drm/radeon/kms: remove benchmarks shorter than one page

copy_blit operation works only on integral number of pages
so benchmarks shorter than one page size (4K) do not make sense

v2: use RADEON_GPU_PAGE_SIZE instead of "magic" 1024 number and
    sweep sizes between 1 * <page_size> to 16K * <page_size> doubling
    the size in each iteration; we get the same coverage, as
    in the original benchmark, but guarantee integer multiples
    of page size

v3: add whitespace between '*' operator per review received from
    zajec5@gmail.com

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>