rust: drm/kms: Add Device::num_crtcs()

A binding for checking drm_device.num_crtcs. We'll need this in a moment
for vblank support, since setting it up requires knowing the number of
CRTCs that a driver has initialized.

Signed-off-by: Lyude Paul <lyude@redhat.com>

rust/kernel/drm/kms.rs

@@ -326,6 +326,20 @@ impl<T: KmsDriver> Device<T> {
         //   prevent errors)
         unsafe { (*self.as_raw()).mode_config.num_total_plane as u32 }
     }
+
+    /// Return the number of registered CRTCs
+    /// TODO: while `num_crtc` is of i32, that type actually makes literally no sense here and just
+    /// causes problems and unecessary casts. Same for num_plane(). So, fix that at some point (we
+    /// will never get n < 0 anyway)
+    #[inline]
+    pub fn num_crtcs(&self) -> u32 {
+        // SAFETY:
+        // * This can only be modified during the single-threaded context before registration, so
+        //   this is safe
+        // * num_crtc could be >= 0, but no less - so casting to u32 is fine (and better to prevent
+        //   errors)
+        unsafe { (*self.as_raw()).mode_config.num_crtc as u32 }
+    }
 }
 
 /// A modesetting object in DRM.