drm/amdgpu: get_fw_version isn't ASIC specific

Method of getting firmware version is the same across ASICs, so remove
them from ASIC-specific files and create one in amdgpu_amdkfd.c. This new
created get_fw_version simply reads fw_version from adev->gfx than parsing
the ucode header.

Signed-off-by: Amber Lin <Amber.Lin@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>