drm/format-helper: Add generic conversion to 8-bit formats

Add drm_fb_xfrm_line_32to8() to implement conversion from 32-bit
pixels to 8-bit pixels. The pixel-conversion is specified by the
given callback parameter. Mark the helper as always_inline to avoid
overhead from function calls.

Then implement all existing line-conversion functions with the new
generic call and the respective pixel-conversion helper.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250328141709.217283-6-tzimmermann@suse.de