drm/print: add drm_debug_enabled()

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

include/drm/drm_print.h

@@ -293,6 +293,11 @@ static inline struct drm_printer drm_err_printer(const char *prefix)
 #define DRM_UT_LEASE		0x80
 #define DRM_UT_DP		0x100
 
+static inline bool drm_debug_enabled(unsigned int category)
+{
+	return unlikely(drm_debug & category);
+}
+
 __printf(3, 4)
 void drm_dev_printk(const struct device *dev, const char *level,
 		    const char *format, ...);