drm/i915: Start passing around i915_vma from execbuffer

During execbuffer we look up the i915_vma in order to reserve them in
the VM. However, we then do a double lookup of the vma in order to then
pin them, all because we lack the necessary interfaces to operate on
i915_vma - so introduce i915_vma_pin()!

v2: Tidy parameter lists to remove one level of redirection in the hot
path.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470324762-2545-15-git-send-email-chris@chris-wilson.co.uk