drm/gma500: fix psb_intel_lvds_mode_valid()'s return type

The method struct drm_connector_helper_funcs::mode_valid is defined
as returning an 'enum drm_mode_status' but the driver implementation
for this method, psb_intel_lvds_mode_valid(), uses an 'int' for it.

Fix this by using 'enum drm_mode_status' for psb_intel_lvds_mode_valid().

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180424131458.2060-1-luc.vanoostenryck@gmail.com