reset_skip_context() was always clearing the entropy contexts for
all three color planes, using a block size that corresponded with
the luma plane.

However, when chroma_2x2 is disabled, then for sub-8x8 luma block
sizes, the corresponding chroma block size is always 4x4, and the
skip flag only affects the chroma blocks corresponding to the
upper-left luma block.

This patch makes reset_skip_context() reset the contexts that
actually correspond to the chroma blocks that are skipped (if any).
It also moves reset_skip_context() to av1_reset_skip_context() in
blockd.c, because blockd.h gets included before onyx_int.h, which
declares the required is_chroma_reference() function.
reset_skip_context() was too large and used in too many places to
be a reasonable candidate for inlining, anyway.

AWCY results on objective-1-fast:

cb4x4-fix-base@2017-05-11T06:26:50.159Z -> cb4x4-fix-reset_skip@2017-05-11T06:28:45.482Z
  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
0.0301 |  0.1068 |  0.1463 |   0.0359 | 0.0260 |  0.0347 |     0.0479

A regression (near the noise range), but without this fix, the line
buffer size required by the entropy contexts will be doubled.

Change-Id: I12fa6e60d9c1c7c85927742775a346ea22b3193f