Looking other drm drivers, there is no the restriction that framebuffer
has only one buffer in .create_handle() callback. They use just first
buffer.

If this limitation is removed, there is no reason keeping buffer count
for framebuffer, so we can remove buf_cnt from struct exynos_drm_fb.
Signed-off-by: NJoonyoung Shim <jy0922.shim@samsung.com>
Reviewed-by: NGustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: NInki Dae <inki.dae@samsung.com>