i965: Fixed the CopyImageSubData for ETC2 on Gen < 8

i965: Fixed the CopyImageSubData for ETC2 on Gen < 8

For CopyImageSubData to copy the data during the 1st draw call, we need
to update the shadow tree right before the rendering.

v2:
  - Added assertion that the miptree doesn't need update at the time we
  update the texture surface. (Nanley Chery)

v3:
  - As we now update the tree before the rendering we don't need to copy
  the data during the unmap anymore. Removed the unnecessary update from
  the intel_miptree_unmap in intel_mipmap_tree.c (Nanley Chery)

v4:
  - Fixed unrelated empty line removal (Nanley Chery)
  - As now the intel_upate_etc_shadow of intel_mipmap_tree.c is only
  called inside its following function, we don't need to declare it at
  the top of the file anymore. (Nanley Chery)

Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>