drm/exynos: remove exynos_drm_fb_set_buf_cnt()

The exynos_drm_fb_set_buf_cnt() is used to set buffer count only in
exynos_drm_fbdev_update(). This patch sets directly buffer count in
exynos_drm_framebuffer_init() without using exynos_drm_fb_set_buf_cnt(),
so there is no any reason to keep exynos_drm_fb_set_buf_cnt().

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>