drm/exynos: fix fimd pixel format setting

This patch fixes wrong pixel format setting.

A pixel format is decided according to bpp and depth, or user-requested
format but fimd driver considered only bpp value to decide a proper pixel
format. So this patch makes a proper pixel format to be set according
to drm_framebuffer's pixel_format which is set by addfb with bpp and
depth, or addfb2 with user-requested format.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>