drm/amd/display: Added Check For dc->res_pool

[WHY]
-To ensure dc->res_pool has been initialized

[HOW]
-Check if dc->res_pool is true in
the if statement

Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Pavle Kotarac <Pavle.Kotarac@amd.com>
Signed-off-by: Jarif Aftab <jaraftab@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>