drm/i915/panel: Split range scaling calculation for readiblity

Split the 64b multiplication from the division so that it doesn't sprawl
across a couple of lines and use mul_u32_u32() instead of open-coding
the 64b conversion.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180214091747.12753-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_panel.c

@@ -419,8 +419,9 @@ static uint32_t scale(uint32_t source_val,
 	source_val = clamp(source_val, source_min, source_max);
 
 	/* avoid overflows */
-	target_val = DIV_ROUND_CLOSEST_ULL((uint64_t)(source_val - source_min) *
-			(target_max - target_min), source_max - source_min);
+	target_val = mul_u32_u32(source_val - source_min,
+				 target_max - target_min);
+	target_val = DIV_ROUND_CLOSEST_ULL(target_val, source_max - source_min);
 	target_val += target_min;
 
 	return target_val;