gallium: rename PIPE_CAP_MAX_FRAMES_IN_FLIGHT to PIPE_CAP_THROTTLE

v2: [ Michel Dänzer ]
* Update src/gallium/docs/source/screen.rst accordingly

Signed-off-by: James Xiong <james.xiong@intel.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com> # v1
Reviewed-by: Marek Olšák <marek.olsak@amd.com> # v1