drm: add gem import interface to support drm-hwc [1/1]

drm: add gem import interface to support drm-hwc [1/1]

PD#SWPL-11320

Problem:
current gem driver have not implementted the import interface.
For drm-hwc in android, gralloc allocate the dumb buffer, it
should use the import interface to import the allocated buffer
to the drm driver.

Solution:
implement the gem import interface

Verify:
g12a-u200

Change-Id: I32f7705fd67853a1000875b2af69fcaf700330e1
Signed-off-by: Ao Xu <ao.xu@amlogic.com>