drm/amd/display: Correct min comp buffer size

[Why]
In 3-way mpo pipes, there is a case that we
overbook the CRB buffer size. At rare instances,
overbooking the crb will cause underflow. This only
happens when det_size changes dynamically
based on pipe_cnt.

[How]
Set min compbuff size to 1 segment when preparing BW.

Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Duncan Ma <duncan.ma@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>