drm/amd/amdgpu/vcn_v1_0: Fix some function naming disparity

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

 drivers/gpu/drm/amd/amdgpu/vcn_v1_0.c:775: warning: expecting prototype for vcn_v1_0_start(). Prototype was for vcn_v1_0_start_spg_mode() instead
 drivers/gpu/drm/amd/amdgpu/vcn_v1_0.c:1111: warning: expecting prototype for vcn_v1_0_stop(). Prototype was for vcn_v1_0_stop_spg_mode() 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>