drm/exynos: remove exynos_drm_fb_get_buf_cnt()

We can get buffer count of framebuffer using drm_format_num_planes(), so
keeping exynos_drm_fb_get_buf_cnt() is unnecessary.

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>