drm/exynos: Use new drm_fb_helper functions

drm/exynos: Use new drm_fb_helper functions

Use the newly created wrapper drm_fb_helper functions instead of calling
core fbdev functions directly. They also simplify the fb_info creation.

v2:
- Remove unnecessary dealloc cmap in error handling path

Cc: Inki Dae <inki.dae@samsung.com>
Cc: Joonyoung Shim <jy0922.shim@samsung.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>