drm/gem-fb-helper: convert to drm device based logging

Prefer drm_dbg_kms() and drm_err() over all other logging.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191210123050.8799-4-jani.nikula@intel.com