drm/i915: Implement new w/a for underruns with wm1+ disabled

drm/i915: Implement new w/a for underruns with wm1+ disabled

The new workaround from the hw team involves leaving WM1
still disabled but programming the blocks value
identically to WM0, and we also need to set the "ignore
lines watermark" bit for WM1.

v2: Fix commit message wording a bit

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190213165424.22904-3-ville.syrjala@linux.intel.com
Reviewed-by: Clint Taylor <Clinton.A.Taylor@intel.com>