gallium: Add a PIPE_CAP_FRAGMENT_SHADER_INTERLOCK.

Corresponding to GL_ARB_fragment_shader_interlock and
GL_NV_fragment_shader_interlock.  Currently, only the NIR paths
support this functionality, but someone could conceivably add it
to TGSI too.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
3 files changed