h264: fix first macroblock bit offset calculation (ILK, SNB, IVB).

h264: fix first macroblock bit offset calculation (ILK, SNB, IVB).

Fix and simplify the scan for emulation_prevention_bytes, thus avoiding
a read beyond the end of the slice data buffer. Besides, if slice_header()
bytes are needed, use dri_bo_get_subdata() instead.

HW specific changes:
- SNB: make the HW skip the emulation prevention bytes itself.
- IVB: fix MFD_AVC_BSD_OBJECT to report the actual slice data buffer size.

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>