drm: Add drm_format_plane_width() and drm_format_plane_height()

Add a few helpers to get the dimensions of the chroma plane(s).

v2: Add kernel-doc (Daniel)
v3: Fix kerneldoc "Returns:" style (Daniel)
    Uninline the functions and check for num_planes (Daniel)
v4: Add the required EXPORT_SYMBOL()s

Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1455031784-10941-1-git-send-email-ville.syrjala@linux.intel.com