drm/i915: Wait for vblank in i9xx_disable_crtc() for gen 2 only

The wait for other gens was added in commit 564ed191 ("drm/i915:
gmch: fix stuck primary plane due to memory self-refresh mode") since
that's necessary when disabling cxsr. However, cxsr disabling was later
moved to intel_pre_disable_primary() in commit 87d4300a ("drm/i915:
Move intel_(pre_disable/post_enable)_primary to intel_display.c, and use
it there.") and that function got its own vblank wait for cxsr in commit
262cd2e1 ("drm/i915: CHV DDR DVFS support and another watermark
rewrite"). So remove the extra vblank wait from i9xx_crtc_distable().

Cc: Kalyan Kondapally <kalyan.kondapally@intel.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1458634284-6080-1-git-send-email-ander.conselvan.de.oliveira@intel.com