drm/format-helper: Add generic conversion to 16-bit formats

Add drm_fb_xfrm_line_32to16() to implement conversion from 32-bit
pixels to 16-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. There's one
pixel-conversion helper that swaps output bytes. It is for gud and
requires refactoring, so don't move it into the header file.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250328141709.217283-5-tzimmermann@suse.de