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

Prefer drm_dbg_kms(), drm_info(), and drm_err() over all other
logging. This is about KMS so switch to the KMS category while at it.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191210123050.8799-3-jani.nikula@intel.com