gallium/radeon: add a new HUD query for the number of mapped buffers

gallium/radeon: add a new HUD query for the number of mapped buffers

Useful when debugging applications which map a ton of buffers
and also because we used to run into Linux's limit on the number
of simultaneous mmap() calls.

v2: - update the commit message

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>