drm/i915/gtt: Make I915_PDPES_PER_PDP inline function

The macro takes a vm pointer at some sites, and dev_priv on others
We were saved as the internal macro never deferences the pointer
given.

As the number of pdpes depend on vm configuration, make it
as a inline function that accepts vm pointer.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wsilon.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1488295691-9404-1-git-send-email-mika.kuoppala@intel.com