drm_hwcomposer: platformhisi: Remove fake-importing

With CanImportBuffer() in place we don't need fake importing
anymore. Buffers should be checked before supplied to ImportBuffer()
and plane planner.

Instead of fake importing return -EINVAL for non HW_FB buffers in
ImportBuffer() and skip non HW_FB in planner. Additionally, return
error from planner if we didn't emplace any layer to force client
compositing.

Signed-off-by: Alexey Firago <alexey_firago@mentor.com>