drm/omap: Use new drm_fb_helper functions

drm/omap: 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.

v3:
- Update error handling for new drm_fb_helper funcs. Check using IS_ERR()
  instead of checking for NULL.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
v2:
- No changes

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>