drm/i915: Split out reading of HSW watermark latency values

Move parsing of MCH_SSKPD to a separate function, we'll add other
platforms there later.

Note: Chris spotted an empty struct initializer and wondered whether
that is hiding a compilier warning. Ville explained that it should
have been part of the patch that extends this function to snb/ivb,
which don't have all levels hsw has. I've figured it's ok to keep it
here with a small note.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Add note about the ominous struct initializer.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>