drm/radeon: Workaround radeon driver bug for Loongson

LoongArch inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5OHOB



--------------------------------

Radeon driver can not handle the interrupt is faster than DMA data, so
irq handler must update an old ih.rptr value in IH_RB_RPTR register to
enable interrupt again when interrupt is faster than DMA data.

Change-Id: If15325e6c22c6e01e0fdf78588f0b4f2fd4ef569
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Zhijie Zhang <zhangzhijie@loongson.cn>