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

Cc: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f6f65ca7e27e949533e8cd1f43c61ecac73c658e.1569329774.git.jani.nikula@intel.com