drm/fb: remove unused function: drm_gem_fbdev_fb_create()

After migrating several drivers to the generic fbdev
emulation there are no users left of drm_gem_fbdev_fb_create.
Delete the function.

Noticed that there was no callers while browsing
around in the drm_fb* code.

The code that referenced the function was removed by:
commit 13aff184 ("drm/qxl: remove dead qxl fbdev emulation code")

The actual use was removed by:
commit 26d4707d

 ("drm/qxl: use generic fbdev emulation")

v2:
- Updated changelog based on feedback from Noralf

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190721140610.GA20842@ravnborg.org