drm/vmwgfx/vmwgfx_kms: Mark vmw_{cursor,primary}_plane_formats as __maybe_unused

drm/vmwgfx/vmwgfx_kms: Mark vmw_{cursor,primary}_plane_formats as __maybe_unused

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/vmwgfx/vmwgfx_kms.h:256:23: warning: ‘vmw_cursor_plane_formats’ defined but not used [-Wunused-const-variable=]
 drivers/gpu/drm/vmwgfx/vmwgfx_kms.h:248:23: warning: ‘vmw_primary_plane_formats’ defined but not used [-Wunused-const-variable=]
 drivers/gpu/drm/vmwgfx/vmwgfx_kms.h:256:23: warning: ‘vmw_cursor_plane_formats’ defined but not used [-Wunused-const-variable=]
 drivers/gpu/drm/vmwgfx/vmwgfx_kms.h:248:23: warning: ‘vmw_primary_plane_formats’ defined but not used [-Wunused-const-variable=]
 drivers/gpu/drm/vmwgfx/vmwgfx_kms.h:256:23: warning: ‘vmw_cursor_plane_formats’ defined but not used [-Wunused-const-variable=]
 drivers/gpu/drm/vmwgfx/vmwgfx_kms.h:248:23: warning: ‘vmw_primary_plane_formats’ defined but not used [-Wunused-const-variable=]

Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Roland Scheidegger <sroland@vmware.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Zack Rusin <zackr@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210115181313.3431493-8-lee.jones@linaro.org