gl_drm: use EGL_IMG_context_priority if available

gl_drm: use EGL_IMG_context_priority if available

This is a hint that we want a high priority context.  Since gl_drm is
likely a compositor or a full screen app, it makes sense that it try to
use this (but other engines probably shouldn't)

Based loosely on Chris Wilson's weston patch to do the same thing.
(weston commit b678befb6ed055e6c66466505d9195a3cebf8073)

As this extension appears to have been around for years, I haven't
added fallback defines for:
EGL_CONTEXT_PRIORITY_LEVEL_IMG  0x3100
EGL_CONTEXT_PRIORITY_HIGH_IMG   0x3101