drm/i915: Use the GM45 VGA hotplug workaround on G45 as well.

Although spec say CRT_HOTPLUG_ACTIVATION_PERIOD_64 is only useful for
mobile platform, it is also required to detect vga on G4x desktops correctly.
Tested on G45/G43/Q45 platforms with no regressions.

It fixed freedesktop.org bug #21120 and part of bug #21210

Signed-off-by: Ma Ling <ling.ma@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>