backend-drm: Check pixel format before constructing the zpos candidate list

backend-drm: Check pixel format before constructing the zpos candidate list

We can determine if the pixel format used by the clients buffer is
scan-out capable much sooner, so do it when constructing the zpos
candidate list. It also removes the checks in their respective
prepare_ functions.

Avoids the situation where we'd need to retrieve the DRM framebuffer each time
when checking the pixel format.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>