drm: Specify a bit more the DRM_CAP_CURSOR_{WIDTH, HEIGHT} caps

Earlier this week, there was a bit of confusion about those new
capabilities, to the point I think it's better to document the intention
and API contract.

The comment documents the current situation:
 - the radeon driver returns the only valid size for the hw
 - i915 returns the maximun cursor size
 - other drivers fall back to returning 64x64

The common contract is to return a valid cursor size.

Cc: Sagar Kamble <sagar.a.kamble@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>