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: f39db26c5428 ("drm: Add kms driver for loongson display controller")
Signed-off-by: Harshit Mogalapalli <[e-mail]>
Reviewed-by: Sui Jingfeng <[e-mail]>
Signed-off-by: Sui Jingfeng <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/loongson/lsdc_pixpll.c

···
 	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;