drm/amd/display: add a license to cursor_reg_cache.h

It's MIT.

Fixes: b73353f7

 ("drm/amd/display: Use the same cursor info across features")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>