xen/x86: obtain full video frame buffer address for Dom0 also under EFI

xen/x86: obtain full video frame buffer address for Dom0 also under EFI

The initial change would not work when Xen was booted from EFI: There is
an early exit from the case block in that case. Move the necessary code
ahead of that.

Fixes: fa779ff427bc ("xen/x86: obtain upper 32 bits of video frame buffer address for Dom0")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/2501ce9d-40e5-b49d-b0e5-435544d17d4a@suse.com
Signed-off-by: Juergen Gross <jgross@suse.com>