i965: Use blorp_copy for doing r8 stencil updates on HSW

i965: Use blorp_copy for doing r8 stencil updates on HSW

The blorp_copy entrypoint is designed for doing memcpy like operations
which is what we need to do here while blorp_blit is for handling format
conversion and scaling.  Using blorp_copy is much simpler and prevents
us from getting formats wrong.  While we're here, we get rid of the
layers_per_blit thing since stencil always uses interleaved MSAA.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>