i965: Only enable oMask output when there's a multisample FBO.

i965: Only enable oMask output when there's a multisample FBO.

The ARB_sample_shading specification says that setting gl_SampleMask
bits to 0 means that the corresponding sample "should be considered
uncovered for the purposes of multisample fragment operations
(Section 4.1.3)."

The OpenGL 4.4 specification, section 17.3.3 ("Multisample Fragment
Operations") specifies:

"No changes to the fragment alpha or coverage values are made at this
 step if MULTISAMPLE is disabled, or if the value of SAMPLE_BUFFERS
 is not one."

oMask output alters coverage masks and can kill pixels.  We need to
disable it in the above case, which conveniently corresponds to
key->multisample_fbo being false.

Khronos bug #12188 also spells this out clearly:
https://cvs.khronos.org/bugzilla/show_bug.cgi?id=12188

Fixes two Piglit tests:
tests/spec/arb_sample_shading/builtin-gl-sample-mask-simple 0
tests/spec/arb_sample_shading/builtin-gl-sample-mask 0

Fixes 21 ES3 conformance tests:
ES31-CTS.sample_variables.mask.rgba8.samples_0.mask_zero
ES31-CTS.sample_variables.mask.rgba8.samples_0.mask_0
ES31-CTS.sample_variables.mask.rgba8.samples_0.mask_1
ES31-CTS.sample_variables.mask.rgba8.samples_0.mask_2
ES31-CTS.sample_variables.mask.rgba8.samples_0.mask_3
ES31-CTS.sample_variables.mask.rgba8.samples_0.mask_7
ES31-CTS.sample_variables.mask.rgba8i.samples_0.mask_zero
ES31-CTS.sample_variables.mask.rgba8i.samples_0.mask_3
ES31-CTS.sample_variables.mask.rgba8i.samples_0.mask_4
ES31-CTS.sample_variables.mask.rgba8i.samples_0.mask_5
ES31-CTS.sample_variables.mask.rgba8i.samples_0.mask_7
ES31-CTS.sample_variables.mask.rgba8ui.samples_0.mask_zero
ES31-CTS.sample_variables.mask.rgba8ui.samples_0.mask_2
ES31-CTS.sample_variables.mask.rgba8ui.samples_0.mask_3
ES31-CTS.sample_variables.mask.rgba8ui.samples_0.mask_4
ES31-CTS.sample_variables.mask.rgba8ui.samples_0.mask_6
ES31-CTS.sample_variables.mask.rgba32f.samples_0.mask_zero
ES31-CTS.sample_variables.mask.rgba32f.samples_0.mask_0
ES31-CTS.sample_variables.mask.rgba32f.samples_0.mask_2
ES31-CTS.sample_variables.mask.rgba32f.samples_0.mask_5
ES31-CTS.sample_variables.mask.rgba32f.samples_0.mask_7

Fixes 9 dEQP-GLES31.functional.shaders.sample_variables tests:
sample_mask.discard_half_per_pixel.default_framebuffer
sample_mask.discard_half_per_pixel.singlesample_rbo
sample_mask.discard_half_per_pixel.singlesample_texture
sample_mask.discard_half_per_sample.default_framebuffer
sample_mask.discard_half_per_sample.singlesample_rbo
sample_mask.discard_half_per_sample.singlesample_texture
sample_mask.discard_half_per_two_samples.default_framebuffer
sample_mask.discard_half_per_two_samples.singlesample_rbo
sample_mask.discard_half_per_two_samples.singlesample_texture

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>