drm/print: Handle NULL drm device in __drm_printk()

drm_{err,warn,...}() use __drm_printk() which takes a drm device pointer and
uses the embedded device pointer to print the device. This facility handles
NULL device pointer, but not NULL drm device pointer. This patch makes
__drm_printk() also handle a NULL drm device pointer. The printed output is
identical to if drm->dev had been NULL.

Signed-off-by: Luben Tuikov <ltuikov89@gmail.com>
Reviewed-by: Jani Nikula <jani.nikula@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231117035427.68125-2-ltuikov89@gmail.com