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>

include/uapi/drm/drm.h

@@ -619,6 +619,15 @@ struct drm_gem_open {
 #define  DRM_PRIME_CAP_EXPORT		0x2
 #define DRM_CAP_TIMESTAMP_MONOTONIC	0x6
 #define DRM_CAP_ASYNC_PAGE_FLIP		0x7
+/*
+ * The CURSOR_WIDTH and CURSOR_HEIGHT capabilities return a valid widthxheight
+ * combination for the hardware cursor. The intention is that a hardware
+ * agnostic userspace can query a cursor plane size to use.
+ *
+ * Note that the cross-driver contract is to merely return a valid size;
+ * drivers are free to attach another meaning on top, eg. i915 returns the
+ * maximum plane size.
+ */
 #define DRM_CAP_CURSOR_WIDTH		0x8
 #define DRM_CAP_CURSOR_HEIGHT		0x9