drm/amd/display: fixed integer types and null check locations

drm/amd/display: fixed integer types and null check locations

[ Upstream commit 0484e05d048b66d01d1f3c1d2306010bb57d8738 ]

[why]:
issues fixed:
- comparison with wider integer type in loop condition which can cause
infinite loops
- pointer dereference before null check

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Josip Pavic <josip.pavic@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Sohaib Nadeem <sohaib.nadeem@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>