drm/exynos/fimc: simplify and rename fimc_dst_get_buf_seq

fimc_dst_get_buf_seq returns number of buffers
so the name should be fimc_dst_get_buf_count.
Function body has been simplified.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>