drm/msm/sde: correct index lookup for interrupt clear register

Interrupt get status function incorrectly uses interrupt index to
lookup clear register offset. Correct get status function to use
register index instead to lookup up interrupt offset register.

CRs-Fixed: 2053107
Change-Id: I0c298e0b2b2cbc19758ff84be35ba2d2ce52aeb3
Signed-off-by: Alan Kwong <akwong@codeaurora.org>