drm/loongson: add kernel modesetting driver support for ls7a1000/ls7a2000

LoongArch inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I6BWFP



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 1) Support Double Screen HW Cursor
 2) Support LS7A1000/LS7A2000 + 3A5000
 3) CRTC's DMA Step is remain 256 bytes

Change-Id: Id3c3a7bebf1e95dcc882f69ba20ff3b7e57d275d
Signed-off-by: Jingfeng Sui <suijingfeng@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>