Similar to 16x16 dequant and idct, based on the value of eobs, the
8x8 dequant and idct calculation was simplified to improve decorder
performance.

Combined vp9_dequant_idct_add_8x8 and vp9_dequant_dc_idct_add_8x8
to eliminate duplicate code.

Change-Id: Ia58e50ab27f7012b7379c495837c9c0b5ba9cf7f