i965: Respect miptree offsets in intel_readpixels_tiled_memcpy()

i965: Respect miptree offsets in intel_readpixels_tiled_memcpy()

Respect intel_miptree_slice::x_offset,y_offset and
intel_mipmap_tree::offset. All three may be non-zero when glReadPixels
is called on an EGLImage created from the non-base slice of a miptree.

Patch 2/2 that fixes test
'dEQP-EGL.functional.image.create.gles2_cubemap_*'.

Reported-by: Haixia Shi <hshi@chromium.org>
Diagnosed-by: Haixia Shi <hshi@chromium.org>
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Change-Id: I4b397b27e55a743a7094d29fb0a6a4b6b34352b0