drm/cirrus: Use new drm_fb_helper functions

drm/cirrus: Use new drm_fb_helper functions

Use the newly created wrapper drm_fb_helper functions instead of calling
core fbdev functions directly.

v3:
- Don't touch remove_conflicting_framebuffers

v2:
- Remove stray goto label out_iounmap

Cc: Thierry Reding <treding@nvidia.com>
Cc: Zach Reizner <zachr@google.com>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>