drm/amd/amdgpu/dce_v6_0: Repair function name of 'si_get_number_of_dram_channels()'

drm/amd/amdgpu/dce_v6_0: Repair function name of 'si_get_number_of_dram_channels()'

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

 drivers/gpu/drm/amd/amdgpu/dce_v6_0.c:468: warning: expecting prototype for cik_get_number_of_dram_channels(). Prototype was for si_get_number_of_dram_channels() 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: Luben Tuikov <luben.tuikov@amd.com>
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>