drm/amd/display: always-true lower-bound assert

drm/amd/display: always-true lower-bound assert

[Why]
A recent type change made the lower-bound part of the OTG instance
assert redundant, which can trigger static-analysis noise and distract
from actionable diagnostics.

[How]
Kept the meaningful upper-bound range validation required for safe
narrowing to uint8_t. Removed the redundant non-negative portion of the
assert so the check matches current type semantics. Revalidated with the
latest debug build log: no warnings and no build-failure markers.

Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Signed-off-by: Gaghik Khachatrian <gaghik.khachatrian@amd.com>
Signed-off-by: James Lin <pinglei.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>