drm/panic: Fix 24bit pixel crossing page boundaries

[ Upstream commit 23437509 ]

When using page list framebuffer, and using RGB888 format, some
pixels can cross the page boundaries, and this case was not handled,
leading to writing 1 or 2 bytes on the next virtual address.

Add a check and a specific function to handle this case.

Fixes: c9ff2808 ("drm/panic: Add support to scanout buffer as array of pages")
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20251009122955.562888-7-jfalempe@redhat.com


Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>