drm/exynos: remove buffer creation of fbdev from drm framebuffer creation

The fbdev fb and the user fb is created from same function -
exynos_drm_fb_create, but this function creates not only drm framebuffer
but buffer of fbdev. Remove it because it complicates codes and use
exynos_drm_gem_create() than exynos_drm_buf_create() to create buffer of
fbdev, it give better consistency of codes and more clear
implementation.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>