drm/i915: Update scanline_offset only for active crtcs

update_scanline_offset() in intel_sanitize_crtc() was supposed to
be called only for active crtcs. But due to some underrun patches it
now gets updated for all crtcs on gmch platforms.

Move the update_scanline_offset() to the very beginning of
intel_sanitize_crtc() where we update the vblank state. This seems like
a better place anyway since the scanline offset ought to be up to date
before we might need to consult it. So before any vblanky stuff happens.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>