drm/panic: Simplify logo handling

Move logo rectangle initialisation, and logo drawing in separate
functions, so they can be re-used by different panic screens.
It prepares the introduction of the QR code panic screen.

Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240822073852.562286-4-jfalempe@redhat.com