Normalize more intrinsic filenames

vp9_dequantize_x86 has only sse2 functions.

vp9_dct_sse2_intrinsics has no namespace collision and can drop
_intrinsics.

vp9_idct_mmx.h is unused.

Change-Id: Ic16e31fb372a1d1e841a62ecb4189fe8f95808ec