drm: Remove drm_num_crtcs() helper

The drm_num_crtcs() helper determines the number of CRTCs by iterating
over the list of CRTCs that have been registered with the mode config.
However, we already keep track of that number in the mode config's
num_crtcs field, so we can simply retrieve the value from that and
remove the extra helper function.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240227112038.411846-1-thierry.reding@gmail.com