i915: Fix off-by-one for drawing rectangle.

i915: Fix off-by-one for drawing rectangle.

The drawing rectangle is given in *inclusive* pixel values, so the range
is only [0,2047]. Hence when rendering to a 2048 wide target, such as an
extended desktop, we would issue an illegal instruction zeroing the draw
area.

Fixes:

  Bug 27408: Primary and Secondary display blanks in extended
             desktop mode with Compiz enabled
  https://bugs.freedesktop.org/show_bug.cgi?id=27408

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>