drm/i915: kill dev_priv->pm.regsave

Now that we don't keep the hotplug interrupts enabled anymore, we can
kill the regsave struct and just cal the normal IRQ preinstall,
postinstall and uninstall functions. This makes it easier to add
runtime PM support to non-HSW platforms.

The only downside is in case we get a request to update interrupts
while they are disabled, won't be able to update the regsave struct.
But this should never happen anyway, so we're not losing too much.

v2: - Rebase.
v3: - Rebase.
v4: - Rebase.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>