drm/i915: Refactor ttm ghost obj detection

Currently i915_ttm_to_gem() returns NULL for ttm ghost
object which makes it unclear when we should add a NULL
check for a caller of i915_ttm_to_gem() as ttm ghost
objects are expected behaviour for certain cases.

Create a separate function to detect ttm ghost object and
use that in places where we expect a ghost obj from ttm.

Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221014131427.21102-1-nirmoy.das@intel.com