drm: make drm_core_check_feature() bool that it is

Bool is the more appropriate return type here, use it.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180327204722.31246-3-jani.nikula@intel.com