drm/hyperv: Remove firmware framebuffers with aperture helper

Replace use of screen_info state with the correct interface from
the aperture helpers. The state is only for architecture and firmware
code. It is not guaranteed to contain valid data. Drivers are thus
not allowed to use it.

For removing conflicting firmware framebuffers, there are aperture
helpers. Hence replace screen_info with the correct function that will
remove conflicting framebuffers for the hyperv-drm driver. Also
move the call to the correct place within the driver.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Helge Deller <deller@gmx.de>