drm/panic: Fix qr_code, ensure vmargin is positive

drm/panic: Fix qr_code, ensure vmargin is positive

Depending on qr_code size and screen size, the vertical margin can
be negative, that means there is not enough room to draw the qr_code.

So abort early, to avoid a segfault by trying to draw at negative
coordinates.

Fixes: cb5164ac43d0f ("drm/panic: Add a QR code panic screen")
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20251009122955.562888-4-jfalempe@redhat.com
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>