drm/mcde: Improve pixel fetcher FIFO depth setting

The pixel fetcher FIFO depth was just hardcoded to 48
which works fine as long as the framebuffer is 32BPP
and the DSI output is RGB888.

We will need more elaborate handling for some buffer
formats and displays, so start to improve this function
by setting reasonable defaults for 32, 24 and 16 BPP
framebuffers.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: newbytee@protonmail.com
Cc: Stephan Gerhold <stephan@gerhold.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200808223122.1492124-1-linus.walleij@linaro.org