igc: Improve the DMA mapping flow

Improve the probe flow and set both the DMA mask and the coherent
to the same thing. Make the flow optimized and cleared.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>