drm/i915: Make __i915_printk debug output behave the same as DRM_DEBUG_DRIVER

Joonas and Daniel remarked that our debugging output should stay compatible
with the core DRM's debug facility. The recently added __i915_printk() would
output debug messages even if debugging is completely disabled via the
drm.debug option. To fix this make __i915_printk behave the same as
DRM_DEBUG_DRIVER in this case.

CC: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
CC: Daniel Vetter <daniel.vetter@ffwll.ch>
CC: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1458572937-21712-1-git-send-email-imre.deak@intel.com