drm/exynos: mixer: cleanup pixelformat handling

Move the defines for the pixelformats that the mixer supports out
of mixer_graph_buffer() to the top of the source.
Then select the mixer pixelformat (pf) in mixer_graph_buffer() based on
the plane's pf (and not bpp).
Also add handling of RGB565 and XRGB1555 to the switch statement and
exit early if the plane has an unsupported pf.

Partially based on 'drm/exynos: enable/disable blend based on pixel
format' by Gustavo Padovan <gustavo.padovan@collabora.co.uk>.

v2: Use the shorter MXR_FORMAT as prefix.
v3: Re-add ARGB8888 because of compatibility reasons
    (suggested by Joonyoung Shim).

Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Acked-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>