nv50/ir: enable early fragment test with explicit user control

nv50/ir: enable early fragment test with explicit user control

This feature can be enabled in two ways: as an optimization and by
explicit user control (with OpenGL 4.2 or ARB_shader_image_load_store).

This makes use of the recent TGSI_PROPERTY_FS_EARLY_DEPTH_STENCIL to
force early fragment tests when needed.

This fixes a bunch of
dEQP-GLES31.functional.image_load_store.early_fragment_tests.* tests.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>