drm/exynos: fb: use drm_format_num_planes to get buffer count

drm/exynos: fb: use drm_format_num_planes to get buffer count

The previous code had some special case handling for the buffer
count in exynos_drm_format_num_buffers().

This code was incorrect though, since this special case doesn't
exist for DRM. It stemmed from the existence of the special NV12M
V4L2 format. NV12 is a bi-planar format (separate planes for luma
and chroma) and V4L2 differentiates between a NV12 buffer where
luma and chroma is contiguous in memory (so no data between
luma/chroma), and a NV12 buffer where luma and chroma have two
explicit memory locations (which is then called NV12M).

This distinction doesn't exist for DRM. A bi-planar format always
explicitly comes with the information about its two planes (even
if these planes should be contiguous).

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Acked-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>