drm/amd/display: fix possible buffer overflow relating to secure display

It is possible that adev->dm.dc->caps.max_links is greater than
AMDGPU_MAX_CRTCS. So, to not potentially access unallocated memory use
adev->mode_info.num_crtc to do the bounds check instead of
adev->dm.dc->caps.max_links.

Fixes: 1b11ff76 ("drm/amd/display: Implement multiple secure display")
Fixes: b8ff7e08 ("drm/amd/display: Fix when disabling secure_display")
Reviewed-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>