drm/vkms: Create helpers macro to avoid code duplication in format callbacks

drm/vkms: Create helpers macro to avoid code duplication in format callbacks

The callback functions for line conversion are almost identical for
some format. The generic READ_LINE macro generate all the required
boilerplate to process a line.

Two overrides of this macro have been added to avoid duplication of
the same arguments every time.

Reviewed-by: Maíra Canal <mcanal@igalia.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Link: https://lore.kernel.org/r/20250703-b4-new-color-formats-v7-1-15fd8fd2e15c@bootlin.com
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>