drm/amdgpu: Fix out-of-bounds read of df_v1_7_channel_number

[ Upstream commit d768394f ]

Check the fb_channel_number range to avoid the array out-of-bounds
read error

Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Reviewed-by: Tim Huang <Tim.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>