DRM: Rename clamp variable

linux/kernel.h has a "clamp" macro, but r300_cmdbuf also uses a variable
with the same name. Right now it doesn't seem to include the header,
but sooner or later someone will. So better rename the variable
now.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>