drm: Enable pr_debug() for drm_printer

pr_debug() is conditionally compiled and requires either
dynamic-debugging to be enabled or for the code to opt-in using #define
DEBUG. Since drm_print provides a central debugging facility using
pr_debug(), make sure it will always produce output.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20171027110602.31519-1-chris@chris-wilson.co.uk