drm/loongson: Fix error handling in lsdc_pixel_pll_setup()

There are two problems in lsdc_pixel_pll_setup()
1. If kzalloc() fails then call iounmap() to release the resources.
2. Both kzalloc and ioremap does not return error pointers on failure, so
   using IS_ERR_OR_NULL() checks is a bit confusing and not very right,
   fix this by changing those to NULL checks instead.

Fixes: f39db26c ("drm: Add kms driver for loongson display controller")
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Signed-off-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Link: https://patchwork.freedesktop.org/patch/msgid/20230720123950.543082-1-harshit.m.mogalapalli@oracle.com

drivers/gpu/drm/loongson/lsdc_pixpll.c

@@ -120,12 +120,14 @@ static int lsdc_pixel_pll_setup(struct lsdc_pixpll * const this)
 	struct lsdc_pixpll_parms *pparms;
 
 	this->mmio = ioremap(this->reg_base, this->reg_size);
-	if (IS_ERR_OR_NULL(this->mmio))
+	if (!this->mmio)
 		return -ENOMEM;
 
 	pparms = kzalloc(sizeof(*pparms), GFP_KERNEL);
-	if (IS_ERR_OR_NULL(pparms))
+	if (!pparms) {
+		iounmap(this->mmio);
 		return -ENOMEM;
+	}
 
 	pparms->ref_clock = LSDC_PLL_REF_CLK_KHZ;