drm/i915: LPT:LP needs port A HPD enabled in both north and south

If the CPU and PCH are on the same package we must enabled the port A
HPD also in the south hotplug register. To identify the package type
we simply look at the PCH type: LPT-H means separate package, and
LPT-LP means multi chip package (MCP).

v2: Add comment and pimp commit message

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>