drm/amd/display: Check null-initialized variables

[ Upstream commit 367cd9ce ]

[WHAT & HOW]
drr_timing and subvp_pipe are initialized to null and they are not
always assigned new values. It is necessary to check for null before
dereferencing.

This fixes 2 FORWARD_NULL issues reported by Coverity.

Reviewed-by: Nevenko Stupar <nevenko.stupar@amd.com>
Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
[Xiangyu: BP to fix CVE: CVE-2024-49898, Minor conflict resolution]
Signed-off-by: Xiangyu Chen <xiangyu.chen@windriver.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>