i965/fs: Add brw_wm_prog_key bit specifying whether FB reads should be coherent.

i965/fs: Add brw_wm_prog_key bit specifying whether FB reads should be coherent.

Some of the following changes in this series are specific to the
non-coherent path, so I need some way to tell whether the coherent or
non-coherent path is in use.  The flag defaults to the value of the
gl_extensions::MESA_shader_framebuffer_fetch enable so that it can be
overridden easily on hardware that supports both framebuffer fetch
extensions in order to test the non-coherent path, like:

 MESA_EXTENSION_OVERRIDE=-GL_EXT_shader_framebuffer_fetch

(Of course trying to force-enable the coherent framebuffer fetch
extension on hardware without native support won't work and lead to
assertion failures).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>