drm: Add helper for conversion from signed-magnitude

CTM values are defined as signed-magnitude values. Add
a helper that converts from CTM signed-magnitude fixed
point value to the twos-complement value used by
drm_fixed.

Reviewed-by: Sebastian Wick <[e-mail]>
Reviewed-by: Louis Chauvet <[e-mail]>
Signed-off-by: Harry Wentland <[e-mail]>
Reviewed-by: Daniel Stone <[e-mail]>
Reviewed-by: Melissa Wen <[e-mail]>
Signed-off-by: Simon Ser <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

include/drm/drm_fixed.h

···
 #define DRM_FIXED_EPSILON	1LL
 #define DRM_FIXED_ALMOST_ONE	(DRM_FIXED_ONE - DRM_FIXED_EPSILON)
 
+/**
+ * @drm_sm2fixp
+ *
+ * Convert a 1.31.32 signed-magnitude fixed point to 32.32
+ * 2s-complement fixed point
+ *
+ * @return s64 2s-complement fixed point
+ */
+static inline s64 drm_sm2fixp(__u64 a)
+{
+	if ((a & (1LL << 63))) {
+		return -(a & 0x7fffffffffffffffll);
+	} else {
+		return a;
+	}
+}
+
 static inline s64 drm_int2fixp(int a)
 {
 	return ((s64)a) << DRM_FIXED_POINT;