drm: remove drm_fb_helper_modinit

drm_fb_helper_modinit has a lot of boilerplate for what is not very
simple functionality.  Just open code it in the only caller using
IS_ENABLED and IS_MODULE, and skip the find_module check as a
request_module is harmless if the module is already loaded (and not
other caller has this find_module check either).

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jessica Yu <jeyu@kernel.org>