intel/blorp: Increase the presision of coordinate transform calculations

intel/blorp: Increase the presision of coordinate transform calculations

The result of this calculation goes into an fma() in the shader and we
would like it to be as precise as possible.  The division in particular
was a source of imprecision whenever dst1 - dst0 was not a power of two.
This prevents regressions in some of the new Vulkan CTS tests for blitting
using a filtering of NEAREST.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>