drm/modes: drop maxPitch from drm_mode_validate_size

Totally unused and actually redundant with maxX for display mode
validation. The fb helper otoh needs to check pitch limits,
but that is delegated into drivers instead.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>