The comparison with the number of elements in array df_v3_7_channel_number
is off-by-one and can produce an array out-of-bounds read if
fb_channel_number is equal to the number of elements of the array. Fix
this by changing the comparison to >= instead of >.

Detected by CoverityScan, CID#1469489 ("Out-of-bounds read")

Fixes: 13b58150

 ("drm/amdgpu/df: implement df v3_6 callback functions (v2)")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>