drm/radeon/kms: cleanup benchmark code

factor out repeated code into functions
fix units in which the throughput is reported (megabytes per second
and megabits per second make sense, others are kind of confusing)
make report more amenable to awk and friends (e.g. whitespace is
always the separator, unit is separated from the number, etc)
add #defines for some hard coded constants

besides "beautification" this reorg is done in preparation
for writing more elaborate benchmarks

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>