drm/etnaviv: use drm_debug_enabled() to check for debug categories

drm/etnaviv: use drm_debug_enabled() to check for debug categories

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

Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Russell King <linux+etnaviv@armlinux.org.uk>
Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: etnaviv@lists.freedesktop.org
Acked-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f202f2cdd7c3176649dadeb48a6da4b208e9e829.1569329774.git.jani.nikula@intel.com