freedreno/kgsl: don't even bother trying CREATE_FD

Don't even bother trying DRM_KGSL_GEM_CREATE_FD.  It hasn't worked since
(afaict) 2.6.35 kernels.  And in some cases seems to cause some
problems.  Instead just allocate a minimum size dummy object (just for
purposes of having a handle) and then mmap the framebuffer as user-mem
(which is deprecated, but seems to still work.. and as far as I can tell
is the best option for now).

Signed-off-by: Rob Clark <robclark@freedesktop.org>