drm/amd/display: Fix mismatch type comparison

[ Upstream commit 26873260 ]

The mismatch type comparison/assignment may cause data loss. Since the
values are always non-negative, it is safe to use unsigned variables to
resolve the mismatch.

Signed-off-by: Navid Assadian <navid.assadian@amd.com>
Reviewed-by: Joshua Aberback <joshua.aberback@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>