drm/core: Calculate bpp in afbc helper

Some drivers (komeda, malidp) don't set anything in cpp. If that is the
case the right value can be inferred from the format. Then the "bpp" member
can be eliminated from struct drm_afbc_framebuffer.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200331155308.6345-3-andrzej.p@collabora.com