Add helper to check if a drm debug category is enabled. Convert drm core
to use it. No functional changes.

v2: Move unlikely() to drm_debug_enabled() (Eric)

v3: Keep unlikely() when combined with other conditions (Eric)

Cc: Eric Engestrom <eric@engestrom.ch>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191001140614.26909-1-jani.nikula@intel.com