drm/amd/display: Check null-initialized variables

[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>