drm/amd/display: Add null checks for 'stream' and 'plane' before dereferencing

In the Linux kernel, the following vulnerability has been resolved:

drm/amd/display: Add null checks for 'stream' and 'plane' before dereferencing

This commit adds null checks for the 'stream' and 'plane' variables in

the dcn30_apply_idle_power_optimizations function. These variables were

previously assumed to be null at line 922, but they were used later in

the code without checking if they were null. This could potentially lead

to a null pointer dereference, which would cause a crash.

The null checks ensure that 'stream' and 'plane' are not null before

they are used, preventing potential crashes.

Fixes the below static smatch checker:

drivers/gpu/drm/amd/amdgpu/../display/dc/hwss/dcn30/dcn30_hwseq.c:938 dcn30_apply_idle_power_optimizations() error: we previously assumed 'stream' could be null (see line 922)

drivers/gpu/drm/amd/amdgpu/../display/dc/hwss/dcn30/dcn30_hwseq.c:940 dcn30_apply_idle_power_optimizations() error: we previously assumed 'plane' could be null (see line 922)