drm/fsl-dcu: remove separate compilation unit for fbdev emulation

The separate file fsl_dcu_drm_fbdev.c only initialized fbdev
emulation which is a one-line operation. There is not much more
code on sight which justifies a separate file, hence call the
initialization helper directly from the drv file.

Signed-off-by: Stefan Agner <stefan@agner.ch>