drm/radeon/kms: clean up multiple crtc handling for evergreen+ (v2)

evergreen+ asics have 2-6 crtcs.  Don't access crtc registers
for crtc regs that don't exist as they have very high latency
and may cause problems on some asics.  The previous code missed
a few cases and was not fine grained enough (missed the 4 crtc
case for example).

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=38800

v2: fix typo noticed by Chris Bandy <cbandy@jbandy.com>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Reviewed-by: Michel Dänzer <michel@daenzer.net>
Tested-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>