H264_Encoding: Add the support of packed slice header to be flexible

H264_Encoding: Add the support of packed slice header to be flexible

Under some encoding scenario, the user hopes to generate the packed slice
header data by themself and then the driver can insert the passed slice
header packed data into the coded clip.
1.The VA_ENC_PACKED_HEADER_SLICE flag is exported and it is treated as optional.
This is to say: if packed slice header data is passed, it will be
inserted directly. If no packed slice header data is passed, the driver will
help to generate it.
2.Another restriction is that the packed slice header data is inserted after
the packed rawdata for one slice. That is to say: If it needs to insert the
packed rawdata and slice header data, the packed rawdata will be inserted
firstly(This is handled by the driver).

Signed-off-by: Zhao, Yakui <yakui.zhao@intel.com>
(cherry picked from commit 00111e8a8bfa67b971419b72577eaa1b9f47bc34)

Conflicts:
	src/gen75_mfc.c
	src/gen8_mfc.c