drm/amd/display: Use res_cap to acquire i2c instead of pipe count

[Why]
We should be using the ddc_num from res_caps. As the
pipe count != number of i2c resources.

[How]
Use ddc_num from res_cap instead of pipe count.

Signed-off-by: Derek Lai <Derek.Lai@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>