intel/blorp_blit: Create structure for src & dst coordinates

intel/blorp_blit: Create structure for src & dst coordinates

This will be useful for splitting blits into smaller sizes.

We also make the coordinates of type double rather than float. Since
we will be splitting and scaling the coordinates, we might require
extra precision in the calculations.

v2:
 * Use double instead of float. (Jason)

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>