intel-gtt: new function intel_gtt_mappable_entries

This implementation is stolen from drm/i915, but is equivalent to
the code sprinkled over intel-gtt.c in the various fetch_size functions.
It's not yet used anywhere, though.

Also introduce intel_gtt_init which only calls intel_gtt_stolen_entries.
Over the course of the next patches, this will grow untill it contains
the complete init sequence starting from the call to gtt_mappable_entries.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>