drm/i915: combine i9xx_get_hpd_pins and pch_get_hpd_pins

These functions are quite similar, so combine them with the use of a new
argument for a function that detects long pulses. This will be also
needed by an upcoming patch adding support for BXT long pulse detection.

No functional change.

v2:
- rebase on top -nightly (Daniel)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Sonika Jindal <sonika.jindal@intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>