drm/amdgpu/display: check if fbc is available in set_static_screen_control (v2)

The value is dependent on whether fbc is available.

v2: only check if num_pipes is valid

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>