The vboxvideo driver is missing a call to remove conflicting framebuffers.

Surprisingly, when using legacy BIOS booting this does not really cause
any issues. But when using UEFI to boot the VM then plymouth will draw
on both the efifb /dev/fb0 and /dev/drm/card0 (which has registered
/dev/fb1 as fbdev emulation).

VirtualBox will actual display the output of both devices (I guess it is
showing whatever was drawn last), this causes weird artifacts because of
pitch issues in the efifb when the VM window is not sized at 1024x768
(the window will resize to its last size once the vboxvideo driver loads,
changing the pitch).

Adding the missing drm_fb_helper_remove_conflicting_pci_framebuffers()
call fixes this.

Changes in v2:
-Make the drm_fb_helper_remove_conflicting_pci_framebuffers() call one of
 the first things we do in our probe() method

Cc: stable@vger.kernel.org
Fixes: 2695eae1 ("drm/vboxvideo: Switch to generic fbdev emulation")
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200325144310.36779-1-hdegoede@redhat.com