freedreno/a6xx: Don't try to do Z-as-RGBA blits for mismatched formats.
Fixes piglit crashes doing glCopyTexSubImage from (for example) PIPE_FORMAT_Z24_UNORM_S8_UINT to PIPE_FORMAT_Z32_FLOAT_S8X24_UINT where, in addition to reading the source Z values incorrectly, we would try to dereference the missing separate stencil of the Z24S8 buffer.
Cc: mesa-stable