drm/imx: hide an unused label

The imx_drm_bind function causes a warning in linux-next when
CONFIG_DRM_FBDEV_EMULATION is not set:

drivers/gpu/drm/imx/imx-drm-core.c: In function 'imx_drm_bind':
drivers/gpu/drm/imx/imx-drm-core.c:441:1: error: label 'err_unbind' defined but not used [-Werror=unused-label]

I don't understand why the warning only showed up now, as the
code has not been modified recently, but there is an obvious
fix in adding another #if for the symbol.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: c1ff5a7a ("drm/imx: Remove local fbdev emulation Kconfig option")
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>