drm/i915: Program SHPD_FILTER_CNT on CNP+

The bspec tells us 'Program SHPD_FILTER_CNT with the "500 microseconds
adjusted" value before enabling hotplug detection' on CNP+.  We haven't
been touching this register at all thus far, but we should probably
follow the bspec's guidance.

The register also exists on LPT and SPT, but there isn't any specific
guidance I can find on how we should be programming it there so let's
leave it be for now.

Bspec: 4342
Bspec: 31297
Bspec: 8407
Bspec: 49305
Bspec: 50473

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127221314.575575-3-matthew.d.roper@intel.com