drm/i915/runtime_pm: Share code to enable/disable PCH reset handshake

Instead of have the same code spread into 4 platforms lets share it.
BXT do not have a PCH so here also handling this case by unseting
RESET_PCH_HANDSHAKE_ENABLE.

v2(Rodrigo):
- renamed to intel_pch_reset_handshake()
- added comment about why BXT need the bit to be unset

v3(Rodrigo and Ville):
- added bool have_pch to intel_pch_reset_handshake()
- added back BXT comment

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180918204714.27306-1-jose.souza@intel.com