drm: mxsfb: Merge mxsfb_set_pixel_fmt() and mxsfb_set_bus_fmt()

The mxsfb_set_pixel_fmt() and mxsfb_set_bus_fmt() functions both deal
with format configuration, are always called in a row from
mxsfb_crtc_mode_set_nofb(), and set fields from the LCDC_CTRL register.
This requires a read-modify-update cycle in mxsfb_set_bus_fmt(). Make
this more efficient by merging them together.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200727020654.8231-21-laurent.pinchart@ideasonboard.com