ipa: libipa: pwl: Make the empty() function inline

The Pwl::empty() function is a one-liner that can be easily optimized by
the compiler given the chance. Make it inline.

While at it, move the function documentation block to match the class
declaration order.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/ipa/libipa/pwl.cpp

@@ -182,6 +182,12 @@ void Pwl::prepend(double x, double y, const double eps)
 		points_.insert(points_.begin(), Point({ x, y }));
 }
 
+/**
+ * \fn Pwl::empty() const
+ * \brief Check if the piecewise linear function is empty
+ * \return True if there are no points in the function, false otherwise
+ */
+
 /**
  * \brief Get the domain of the piecewise linear function
  * \return An interval representing the domain
@@ -203,15 +209,6 @@ Pwl::Interval Pwl::range() const
 	return Interval(lo, hi);
 }
 
-/**
- * \brief Check if the piecewise linear function is empty
- * \return True if there are no points in the function, false otherwise
- */
-bool Pwl::empty() const
-{
-	return points_.empty();
-}
-
 /**
  * \brief Evaluate the piecewise linear function
  * \param[in] x The x value to input into the function