drm: use drm_dev_has_vblank more

For historical reasons it's called dev->num_crtcs, which is rather
confusing ever since kms was added. But now we have a nice helper, so
let's use it for better readability!

Only code change is in atomic helpers: vblank support means that
dev->irq_enabled must be set too. Another one of these quirky things
... But since it's implied we can simplify that check.

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200527111134.1571781-1-daniel.vetter@ffwll.ch