drm/vkms: Round fixp2int conversion in lerp_u16

fixp2int always rounds down, fixp2int_ceil rounds up. We need
the new fixp2int_round.

Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241220043410.416867-3-alex.hung@amd.com


Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>