drm/omap: fix operation without fbdev

omapdrm should work fine even if fbdev is missing. The current driver
crashes in that case, though, as it is missing checks for the fbdev.

Add the checks so that we don't free fbdev or restore fbdev mode when
there's no fbdev.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>