drm/amd/display: Use pipe_count for num of opps

[Why]
There is one opp per pipe. For certain RN parts, the fourth pipe is disabled, so there is no opp for it.
res_cap->num_opp is hardcoded to 4, so if we use that to iterate over opps we will crash.

[How]
Use the pipe_count value instead, which is not hardcoded and so will have the correct number.

Signed-off-by: Noah Abradjian <noah.abradjian@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>