drm/imx: fix build failure without CONFIG_DRM_FBDEV_EMULATION

The variable is declared in an #ifdef section, but the user is
now unconditional, which leads to a build failure:

drivers/gpu/drm/imx/imx-drm-core.c: In function 'imx_drm_bind':
drivers/gpu/drm/imx/imx-drm-core.c:264:6: error: 'legacyfb_depth' undeclared (first use in this function); did you mean 'lockdep_depth'?

Remove the remaining #ifdef as well.

Fixes: f53705fd ("drm/imx: Use drm_fbdev_generic_setup()")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180926193846.2490574-1-arnd@arndb.de