drm/tilcdc: Choose console BPP that supports RGB

Choose console BPP that supports RGB and remove the old fbdev bpp
selection code. LCDC on AM335x has red and blue wires switched between
24 bit and 16 bit colors. If 24 format is wired for RGB colors, the 16
bit format is wired for BGR. drm_fbdev_cma_init() does not currently
like anything else but RGB formats, so we must choose such bytes per
pixel value that supports RGB.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>