The firmware may have set up the pipe correctly, but the FIFO
underrun and CRC interrupts are likely not enabled.

This resulted in debugfs_test.read_all_entries failing on haswell,
because of a timeout when reading the crc debugfs entry.

Solve this by enabling FIFO underrun reporting after the initial
fastset, which lets interrupts be generated as expected.

Changes since v1:
- Always enable CPU FIFO underrun reporting for >GEN2,
  and handle GEN2 correctly.
Changes since v2:
- Remove unneeded HAS_DDI, simplify GEN2 case.
Changes since v3:
- Use intel_crtc_pch_transcoder to determine pch transcoder for underruns. (Ville)
- Remove crtc->config dereference in intel_crtc_pch_transcoder. (Ville)

Testcase: debugfs_test.read_all_entries
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171113144043.58658-1-maarten.lankhorst@linux.intel.comReviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>