drivers/gpu/drm/radeon/radeon_combios.c: fix warning

drivers/gpu/drm/radeon/radeon_combios.c: In function 'radeon_combios_get_lvds_info':
drivers/gpu/drm/radeon/radeon_combios.c:893: warning: comparison is always false due to limited range of data type

Cc: Dave Airlie <airlied@linux.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>