virgl: enable FBFETCH if virglrenderer supports it

virgl: enable FBFETCH if virglrenderer supports it

This fixes the following dEQP-GLES31 cases from NotSupported to
Pass for me:

- dEQP-GLES31.functional.blend_equation_advanced.state_query.*
- dEQP-GLES31.functional.blend_equation_advanced.basic.*
- dEQP-GLES31.functional.blend_equation_advanced.srgb.*
- dEQP-GLES31.functional.blend_equation_advanced.msaa.*
- dEQP-GLES31.functional.blend_equation_advanced.barrier.*
- dEQP-GLES31.functional.draw_buffers_indexed.overwrite_*advanced_blend_eq*
- dEQP-GLES31.functional.state_query.indexed.blend_equation_advanced_*
- dEQP-GLES31.functional.debug.negative_coverage.*.advanced_blend.*

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>