drm: Describe the Rotation property bits.

Adds clarification of the rotation property bits. I.e. rotation is
counter clockwise and that reflects are applied before any rotation.

v2: Refer to the define names instead of the property values.
Signed-off-by: Robert Fekete <robert.fekete@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

include/drm/drm_crtc.h

@@ -85,7 +85,11 @@ static inline uint64_t I642U64(int64_t val)
 	return (uint64_t)*((uint64_t *)&val);
 }
 
-/* rotation property bits */
+/*
+ * Rotation property bits. DRM_ROTATE_<degrees> rotates the image by the
+ * specified amount in degrees in counter clockwise direction. DRM_REFLECT_X and
+ * DRM_REFLECT_Y reflects the image along the specified axis prior to rotation
+ */
 #define DRM_ROTATE_MASK 0x0f
 #define DRM_ROTATE_0	0
 #define DRM_ROTATE_90	1