drm/msm: Fix possible uninitialized access in fbdev

Do not run drm_fb_helper_unprepare() if fbdev allocation fails. Avoids
access to an uninitialized pointer. Original bug report is at [1].

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 3fb1f62f

 ("drm/fb-helper: Remove drm_fb_helper_unprepare() from drm_fb_helper_fini()")
Link: https://lore.kernel.org/oe-kbuild-all/202302220810.9dymwCQ8-lkp@intel.com/ # 1
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230222123712.5049-1-tzimmermann@suse.de