drm/rockchip: 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.

This is an effort to create a top level drm fbdev emulation option.

Cc: Mark Yao <mark.yao@rock-chips.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Daniel Kurtz <djkurtz@chromium.org>

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>