drm/radeon: fix card_posted check for newer asics

commit 09fb8bd1a63b0f9f15e655c4fe8d047e5d2bf67a upstream.

Newer asics have variable numbers of crtcs.  Use that
rather than the asic family to determine which crtcs
to check.  This avoids checking non-existent crtcs or
missing crtcs on certain asics.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>