drm/virtio: 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:
- Don't touch remove_conflicting_framebuffers

v2:
- add missing header for virtgpu_fb.c

Cc: David Airlie <airlied@linux.ie>
Cc: Gerd Hoffmann <kraxel@redhat.com>

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