drm/amd/display: Check pipe_ctx before it is used

resource_get_odm_slice_count and resource_get_otg_master_for_stream can
return null, and their returns must be checked before used.

This fixes 4 NULL_RETURNS issues reported by Coverity.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>