drm/stm: Fix an error handling path in stm_drm_platform_probe()

drm/stm: Fix an error handling path in stm_drm_platform_probe()

If drm_dev_register() fails, a call to drv_load() must be undone, as
already done in the remove function.

Fixes: b759012c5fa7 ("drm/stm: Add STM32 LTDC driver")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20fff7f853f20a48a96db8ff186124470ec4d976.1704560028.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>