Code cleanup inside vp9_decodeframe.c.

Removing unused DEC_DEBUG define and dec_debug variable. Changing function
signatures to eliminate code duplication, renaming function
mb_init_dequantizer to init_dequantizer. Also removing redundant curly
braces, and comments.


Change-Id: Ia56ee1b0be5f24abb0e878581845be8a4773c298