i965/blorp: Fix narrowing warnings.

i965/blorp: Fix narrowing warnings.

Blorp has to convert rectangle coordinates from integers to floats in
order to send them down the GPU pipeline.  Recent versions of GCC
issue a warning for this, since a float is not capable of precisely
representing all possible 32-bit integer values.  Suppress the warning
with an explicit type cast in the case of blorp, since rectangle
coordinates will never be large enough to cause a loss of precision.

Reviewed-by: Eric Anholt <eric@anholt.net>