drm/amdgpu: optimize code about format string in gfx_v10_0_init_microcode()

the memset() and snprintf() is not necessary.

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>