drm: Check user mode against overflows

The internal mode representation drm_display_mode uses signed data
types. When converting the user mode to internal representation,
check that the unsigned values don't overflow the signed datatypes.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>