drm: add DRM_CAPs for cursor size

Some hardware may not support standard 64x64 cursors.  Add
a drm cap to query the cursor size from the kernel.  Some examples
include radeon CIK parts (128x128 cursors) and armada (32x64 or 64x32).
This allows things like device specific ddxes to remove asics specific
logic and also allows xf86-video-modesetting to work properly with hw
cursors on this hardware. Default to 64 if the driver doesn't specify
a size.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>