i965: Pass the EGL/DRI context priority through to the kernel

i965: Pass the EGL/DRI context priority through to the kernel

Decode the EGL/DRI priority enum into the [-1023, 1023] range as
interpreted by the kernel and call DRM_I915_GEM_CONTEXT_SETPARAM to
adjust the priority. We use 0 as the default medium priority (also the
kernel default) and so only need adjust up or down. By only doing the
adjustment if not setting to medium, we can faithfully report any error
whilst setting without worrying about kernel version.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>