drm/i915: improve GEN7_ERR_INT clearing for fifo underrun reporting

Same treatment as for SERR_INT: If we clear only the bit for the pipe
we're enabling (but unconditionally) then we can always check for
possible underruns after having disabled the interrupt. That way pipe
underruns won't be lost, but at worst only get reported in a delayed
fashion.

v2: The same logic bug as in the SERR handling change also existed
here. The same bugfix of only reporting missed underruns when the
error interrupt was masked applies, too.

v3: Do the same fixes as for the SERR handling that Paulo suggested in
his review:
- s/%i/%c/ fix in the debug output
- move the DE_ERR_INT_IVB read into the respective if block

Cc: Paulo Zanoni <przanoni@gmail.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Fix up the checkpatch bikeshed Paulo noticed.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>