drm/vblank: use drm_* functions for logging

Replace all DRM_* logging functions with their drm_ counterparts.
checkpatch emits a few "quoted string split across lines",
which is left as is. The strings was already split in the original code
base and it would not increase readability to fix them.

v2:
  - added braces to if statement (Thomas)
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200523071224.1716837-2-sam@ravnborg.org