The problem was that some functions were using scale_chroma_bsize()
function to turn sub-8x8 'bsize' to 8x8 'bsize', and then the modified
'bsize' was being passed to rd_pick_intra_sbuv_mode() for example.

In such cases, we cannot rely on the 'bsize' value passed to the
function; instead, we need to look at the original mbmi->sb_type
directly.

Also:
- Added created a common function can_use_palette() to refactor this
logic into one place.
- Added more asserts to easily catch such coding errors in future.

BUG=aomedia:688

Change-Id: I2e9f20c8c5fbc4b3ff41b703a91a02758c3c632f