drm/i915: Tidy i915_gem_valid_gtt_space()

We can replace a couple of tests with an assertion that the passed in
node is already allocated (as matches the existing call convention) and
by a small bit of refactoring we can bring the line lengths to under
80cols.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161205142941.21965-3-chris@chris-wilson.co.uk