radeon: Reduce number of radeon_bo_map calls that require kernel entry.

radeon: Reduce number of radeon_bo_map calls that require kernel entry.

DMA buffers are often remapped many times for singel cs. To reduce number of
mapping calls ending to kernel dma buffers are mapped when allocated and unmapped
when full or in cs flush. This makes indifual mapping calls in other parts of
code simple increment/decrement reference count which is faster than entering
kernel.

This improves Anholt's openarena benchmark from 36 fps to 44 fps.