[NORMATIVE] Correction to EXPLICIT_ORDER_HINT
[NORMATIVE] Correction to EXPLICIT_ORDER_HINT

Prior to this patch, the wrong buffer was being reset in
set_planes_to_neutral_grey() in read_uncompressed_header(). This patch
passes the correct buffer to set_planes_to_neutral_grey() (and in doing
so, slightly alters the function's signature).

BUG=aomedia:1691

Change-Id: I24f716ac4616305b2a5603f452f7c9b6f4e9d582

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom