drm/exynos: check NV12M format specific to Exynos properly

this patch adds buf_cnt variable in exynos_drm_fb structure and
that means a buffer count to drm framebuffer and also adds two
functions to get/set the buffer count from/to exynos_drm_fb structure.
if pixel format is not DRM_FORMAT_NV12MT then it gets a buffer count
to drm framebuffer refering to mode_cmd->handles and offsets.
but when booted, the buffer count will always be 1 because pixel
format of console framebuffer is RGB format.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>