drm/format-helper: Move helpers for pixel conversion to header file

drm/format-helper: Move helpers for pixel conversion to header file

[ Upstream commit c46d18f98261d99711003517c444417a303c7fae ]

The DRM draw helpers contain format-conversion helpers that operate
on individual pixels. Move them into an internal header file and adopt
them as individual API. Update the draw code accordingly. The pixel
helpers will also be useful for other format conversion helpers.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250328141709.217283-2-tzimmermann@suse.de
Stable-dep-of: 05663d88fd0b ("drm/tests: Fix drm_test_fb_xrgb8888_to_xrgb2101010() on big-endian")
Signed-off-by: Sasha Levin <sashal@kernel.org>