drm/i915/display: start a buffer object abstraction layer

drm/i915/display: start a buffer object abstraction layer

The display code needs to deal with gem objects, and mostly uses struct
drm_i915_gem_object. That's not great, because for xe we need to
redefine it struct xe_bo during build.

Start a common interface using struct drm_gem_object, with separate
implementations for i915 and xe. For starters, convert i9xx_wm.c to use
it.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/df6867523a0b5fdd4eb63f657f545603ae6f6e0b.1726589119.git.jani.nikula@intel.com