drm/gma500: fbdev: Calculate buffer geometry with format helpers

drm/gma500: fbdev: Calculate buffer geometry with format helpers

Replace the geometry and size calculation in gma500's fbdev emulation
with DRM format helpers. This consists of 4CC lookup from the fbdev
parameters, format loockup, pitch calculation and size calculation.
Then allocate the GEM buffer object for the framebuffer memory from
the calculated size.

As before, fallback to 16-bit colors if the stolen memory is insufficient
for 32-bit colors. But look at the result from psb_gem_create() instead
of guessing before the allocation. The new method is more reliable when
others allocate from stolen video memory (e.g., cursors).

Set up mode_cmd with the calculated values just before allocating the
framebuffer. This code will later be replaced with a DRM client buffer.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patch.msgid.link/20260206133458.226467-4-tzimmermann@suse.de