vidc: Use O_SYNC for pmem device.

Necessary as usageflags are indicates an uncached
device. Without O_SYNC buffers become corrupt leading to
HW_ERRORS in vidc.

This commit will fix the video playback issues.

A BIG thanks to ARCO for this commit !!!

Change-Id: I2fe8d1ce6b8aa93c608c0fb41f94b6f0484a42c2