drm/amd/amdgpu/gmc_v7_0: Fix potential copy/paste issue

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/amd/amdgpu/gmc_v7_0.c:526: warning: expecting prototype for gmc_v8_0_set_fault_enable_default(). Prototype was for gmc_v7_0_set_fault_enable_default() instead

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>