Remove allocation of > 2kbyte buffers into context memory in
swr_copy_to_scatch_space() (which is used to copy small vertex/index buffers
and shader constants to a scratch space to be used by the upcoming draw.)

Large shader constant allocations need to be done in the circular scratch
buffer instead of context memory, because their values persist across
render calls.

Also lower SCRATCH_SINGLE_ALLOCATION_LIMIT to 8k, since allocations of larger
buffers will get too large for the circular scratch space.

Fixes render issues with CEI Ensight.

Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>