drm/i915: Enable FIFO underrun reporting after initial fastset, v4.

drm/i915: Enable FIFO underrun reporting after initial fastset, v4.

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.com
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>