egl: Support IMG_context_priority

egl: Support IMG_context_priority

IMG_context_priority
https://www.khronos.org/registry/egl/extensions/IMG/EGL_IMG_context_priority.txt

    "This extension allows an EGLContext to be created with a priority
    hint. It is possible that an implementation will not honour the
    hint, especially if there are constraints on the number of high
    priority contexts available in the system, or system policy limits
    access to high priority contexts to appropriate system privilege
    level. A query is provided to find the real priority level assigned
    to the context after creation."

The extension adds a new eglCreateContext attribute for choosing a
priority hint. This stub parses the attribute and copies into the base
struct _egl_context, and hooks up the query similarly.

Since the attribute is purely a hint, I have no qualms about the lack of
implementation before reporting back the value the user gave!

v2: Remember to set the default ContextPriority value to medium.
v3: Use the driRendererQuery interface to probe the backend for
supported priority values and use those to mask the EGL interface.
v4: Treat the priority attrib as a hint and gracefully mask any requests
not supported by the driver, the EGLContext will remain at medium
priority.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Emil Velikov <emli.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>