Allow better abstraction of the drm_debug global variable in the
future. No functional changes.

v2: Move unlikely() to drm_debug_enabled()

Cc: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <sean@poorly.run>
Cc: linux-arm-msm@vger.kernel.org
Cc: freedreno@lists.freedesktop.org
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/c7142cdebb5f6fed527272b333cd6c43c0aa68ec.1569329774.git.jani.nikula@intel.com