i965: Report supported context priorities to EGL/DRI

i965: Report supported context priorities to EGL/DRI

Hook up the RendererQuery for __DRI2_RENDERER_HAS_CONTEXT_PRIORITY to
report the available DRM_I915_GEM_CONTEXT_SETPARAM options based on the
per-client default context. The kernel will validate the request to change
the property, so we get an accurate reflection of available support
(based on kernel version and privilege) and we should only have to do it
once during screen setup -- although the SETPARAM should be fast, they
are still an ioctl each.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>