drmModeAddFB2 allows userspace to specify a real format enum on
non-ancient kernels, as an improvement over the legacy drmModeAddFB
which derives format from a fixed depth/bpp mapping.
As an optimisation, Weston used to decide at the first failure of
drmModeAddFB2 that the ioctl was unavailable: as non-existent DRM
ioctls return -EINVAL rather than -ENOSYS or similar, bad parameters are
not distinguishable from the ioctl not being present.
Mutter has also implemented the same optimisation for dumb framebuffers,
which potentially papers over errors for the gain of avoiding one ioctl
which will rapidly fail on ancient kernels. Remove the optimisation and
always use AddFB2 where possible.
Closes: #14
f2dea5d139