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.