drm/amdgpu/gmc: fix compiler errors [-Werror,-Wmissing-braces] (V2)

Initializing structures with { } is known to be problematic since
it doesn't necessararily initialize all bytes, in case of padding,
causing random failures when structures are memcmp().

This patch fixes the structure initialisation related compiler
error by memset().

V2: rectified missing piece in coding

Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>