drm/loongson: using hdmi hot plug status register

LoongArch inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6ONR1



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remove ugly workaround, don't create platform device anymore

Signed-off-by: Sui Jingfeng <15330273260@189.cn>
Change-Id: I7934f66c8993b4d9d77a9fe04438849e1f6191ac