drm/i915: PSR also doesn't have link_entry_time on SKL.

This bit is also reserved on Skylake. Actually the only
platform that supports this is Haswell, so let's fix
this logic and apply this link entry time only for the
platform that supports it, i.e. Haswell.

This also changes the style to let more clear platform
differences outside the reg write. We would probably catch
this case sooner if separated, or not...

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449880291-21388-1-git-send-email-rodrigo.vivi@intel.com