Improved vp8dx_decode_bool

Relocated the vp8dx_bool_decoder_fill() call, allowing
the compiler to produce better assembly code.  Tests
showed a 1 - 2 % performance boost (x86 using gcc)
for the 720p clip used.

Change-Id: Ic5a4eefed8777e6eefa007d4f12dfc7e64482732