drm/vkms: Fix cpu_to_le16()/le16_to_cpu() warnings

Building with Sparse enabled prints this warning for cpu_to_le16()
calls:

    warning: incorrect type in assignment (different base types)
        expected unsigned short [usertype]
        got restricted __le16 [usertype]

And this warning for le16_to_cpu() calls:

    warning: cast to restricted __le16

Declare the target buffer as __le16 to fix both warnings.

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Acked-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240716161725.41408-2-jose.exposito89@gmail.com