drm/i915: make PDE|PTE platform specific

Nothing outside of i915_gem_gtt.c and more specifically, the relevant
gen specific init function should need to know about number of PDEs, or
PTEs per PD. Exposing this will only lead to circumventing using the
upcoming VM abstraction.

To accomplish this, move the defines into the .c file, rename the PDE
define to be GEN6, and make the PTE count less of a magic number.

The remaining code in the global gtt setup is a bit messy, but an
upcoming patch will clean that one up.

v2: Don't hardcode number of PDEs (Daniel + Jesse)
Reworded commit message to reflect change.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>