drm/i915: correct the calculation of first_pd_entry_in_global_pt

drm/i915: correct the calculation of first_pd_entry_in_global_pt

When ppgtt is enabled, dev_priv->gtt.total has excluded the gtt space
occupied by ppgtt table in i915_gem_init_global_gtt() function. So the
calculation of first_pd_entry_in_global_pt doesn't need to subtract
I915_PPGTT_PD_ENTRIES again. Or else PPGTT directory table will be
destroyed by global gtt allocation.

This regression has been introduced in

commit a54c0c279f3864171fe53c66e769d5a137c5c651
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Thu Jan 24 14:45:00 2013 -0800

    drm/i915: remove intel_gtt structure

The breakage is pretty subtile since the old gtt_total_entries
included the pde range, whereas the new on did not.

Cc: stable@vger.kernel.org
Signed-off-by: Xiong Zhang<xiong.y.zhang@intel.com>
[danvet: Add regression citation and cc: stable. Thanks to Chris for
correcting my wrong guess about which commit broke things.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>