drm:update the return type of drm_virtcon_mode_valid function

When Control Flow Integrity (CFI) is enabled, making a direct
function call using an incompatible function pointer results
in a CFI failure.

The return type of the drm_virtcon_mode_valid function must
match the return type specified in struct
drm_connector_helper_funcs->mode_valid.

Signed-off-by: Quoc Khanh Le <QuocKhanh.Le@arm.com>