drm/fb-helper: improve documentation and print warnings

Clarify the relation between drm_fb_helper_fbdev_setup/teardown. Clarify
requirements for the new generic fbdev emulation API and log some more
details in case the driver does something wrong. Fix related typos.

Cc: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20180906221810.20170-5-peter@lekensteyn.nl
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>