drm/i915: Make g4x_fixup_plane() operational again

Don't enable the cursor until g4x_fixup_plane() had a chance to do
cast its magic spell.

Egbert writes:
"Today I had the chance to test this. First I tried
 if I can still reproduce the blank with this patch
 added when I disable my voodoo g4x_fixup_plane():
 It turned out it still happens however very rarely
 (like 1 out of 20 tries). When I reenabled my voodoo
 the issue still occurred.
 I had to switch two lines around, ie:

         intel_enable_plane(dev_priv, plane, pipe);
         if (IS_G4X(dev))
                 g4x_fixup_plane(dev_priv, pipe);
 +       intel_crtc_update_cursor(crtc, true);

 to avoid the blank screen issue - which is it didn't
 happen in ~75 tries."

v2: Add a comment to remind people of the ordering constraints

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