drm/i915: Use intel_fb_gtt_offset() also for gen2/3 primary plane

drm/i915: Use intel_fb_gtt_offset() also for gen2/3 primary plane

The code to determine the primary plane offset for gen2/3 looks
different than the code for gen4+, but in fact it's doing the same
thing. Let's make it uniform. Allows us to eliminate the 'obj' from
the list of local variables as well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1478550057-24864-6-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>