drm/format-helper: Optimize 32-to-8-bpp conversion

For ease of implementation, existing line-conversion functions
for 8-bit formats write each pixel individually. Optimize the
performance by writing multiple pixels in a single 32-bit store.

v2:
- simplify address calculation (Jani)
- fix typo in commit message (Jocelyn)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250328141709.217283-9-tzimmermann@suse.de