Rename i2f() to int2float(), and make it global so one copy can be removed.

Remove the copy of i2f() in r600_blit_kms.c
We rename the function to something longer now that it is a global
symbol.  This reduces the likelyhood of unintended clashes later.

This might be a candidate for inclusion inside general drm infrastructure.
However, at the moment only the radeon driver uses it.

Signed-off-by: Steven Fuerst <svfuerst@gmail.com>