drm/qxl: use qxl_num_crtc directly

qdev->monitors_config->max_allowed is effectively set by the
qxl.num_heads module parameter, stored in the qxl_num_crtc variable.
Lets get rid of the indirection and use the variable qxl_num_crtc
directly.  The kernel doesn't need to dereference pointers each time it
needs the value, and when reading the code you don't have to trace where
and why qdev->monitors_config->max_allowed is set.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190118122020.27596-16-kraxel@redhat.com