drm/radeon/kms: a few nits

[Ilija Hadzic]

The following three patches address various minor nits. They are all safe
and I have been running with them for several months on a wide variety
of AMD GPUs

The first patch:
[PATCH 1/3] drm/radeon/kms: use crtc-specific dpms functions in
does not change the functionality and affects the function that is used
only by r100 ASICs, but make the code more correct (strictly speaking).

The second patch:
[PATCH 2/3] drm/radeon/kms: fix the crtc number check
makes the crtc number check consistent with the way it is done
in the rest of the driver code without affecting the functionality.
It was actually sent a few months ago and reviewed, but apparently
forgotten (cf. http://lists.freedesktop.org/archives/dri-devel/2011-May/010935.html)

The third patch:
[PATCH 3/3] drm/radeon/kms: use num_crtc instead of hard-coded value
Prevents a possible trouble waiting to happen if AMD ever makes a GPU with
more than 6 CRTCs. I don't know what's cooking in their kitchen but
if such a GPU ever comes out, this will prevent at least one regression ;-)