Use switch - case to downshift from the current link rate. It's a small
loop now, so fine to be replaced with switch - case. With a loop, it is
confusing and hard to follow as reported below.

The patch d76271d2: "drm: xlnx: DRM/KMS driver for Xilinx ZynqMP
DisplayPort Subsystem" from Jul 7, 2018, leads to the following
static checker warning:

	drivers/gpu/drm/xlnx/zynqmp_dp.c:594 zynqmp_dp_mode_configure()
	error: iterator underflow 'bws' (-1)-2
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1596065445-4630-1-git-send-email-hyun.kwon@xilinx.com