drm/mxsfb: improve clk handling for axi clk

Ignoring errors from devm_clk_get() is wrong. To handle not all platforms
having an axi clk use devm_clk_get_optional() instead and do proper error
handling.

Also the clk API handles NULL as a dummy clk (which is also returned by
devm_clk_get_optional() if there is no clk) so there is no need to check
for NULL before calling clk_prepare_enable() or its counter part.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200720153254.18071-1-u.kleine-koenig@pengutronix.de

drivers/gpu/drm/mxsfb/mxsfb_drv.c

@@ -78,13 +78,11 @@ static const struct mxsfb_devdata mxsfb_devdata[] = {
 
 void mxsfb_enable_axi_clk(struct mxsfb_drm_private *mxsfb)
 {
-	if (mxsfb->clk_axi)
 	clk_prepare_enable(mxsfb->clk_axi);
 }
 
 void mxsfb_disable_axi_clk(struct mxsfb_drm_private *mxsfb)
 {
-	if (mxsfb->clk_axi)
 	clk_disable_unprepare(mxsfb->clk_axi);
 }
 
@@ -235,9 +233,9 @@ static int mxsfb_load(struct drm_device *drm,
 	if (IS_ERR(mxsfb->clk))
 		return PTR_ERR(mxsfb->clk);
 
-	mxsfb->clk_axi = devm_clk_get(drm->dev, "axi");
+	mxsfb->clk_axi = devm_clk_get_optional(drm->dev, "axi");
 	if (IS_ERR(mxsfb->clk_axi))
-		mxsfb->clk_axi = NULL;
+		return PTR_ERR(mxsfb->clk_axi);
 
 	mxsfb->clk_disp_axi = devm_clk_get(drm->dev, "disp_axi");
 	if (IS_ERR(mxsfb->clk_disp_axi))