drm/vc4: Use 16bpp by default for the fbdev buffer

The preferred bpp for the fbdev emulation buffer has been 32 so far, which
means that by default we will allocate an 8MB buffer with a 1920x1080
resolution.

Worse this memory will be allocated from the CMA pool, and will never be
freed even if we don't use the fbdev emulation. Therefore, reducing it is a
big deal, and switching to 16bpp by default will gain us around 4MB at
1920x1080, while keeping decent color depth. And users still have the
option to switch to 32bpp using the kernel command line.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190306140245.21973-1-maxime.ripard@bootlin.com