drm/radeon: Add implementation of get_fw_version

This patch implements a new interface that was added to the kfd-->kgd interface.
The new interface function retrieves the firmware version that is currently
in use by a specific engine. The firmware was uploaded to the engine by the
radeon driver.

v2: Returns the fw version of the specific engine, as passed into the function
by a new parameter

Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>