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