drm/exynos: remove buf_cnt from struct exynos_drm_fb

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: Joonyoung Shim <jy0922.shim@samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>