drm_hwcomposer: Don't try to import client layer in headless mode

Pipeline is null in the headless mode, therefore any attempt to
import the buffer causes a null-pointer dereference crash.

Fixes: 4b2cc484 ("drm_hwcomposer: Reorganize struct DrmHwcLayer")
Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>