drm/amdgpu: mark gfx_v9_4_3_disable_gpa_mode() static

This was left global by accident, the corresponding functions for other hardware types are already static:

drivers/gpu/drm/amd/amdgpu/gfx_v9_4_3.c:1072:6: error: no previous prototype for function 'gfx_v9_4_3_disable_gpa_mode' [-Werror,-Wmissing-prototypes]

Fixes: 86301129 ("drm/amdgpu: split gc v9_4_3 functionality from gc v9_0")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>