drm: msm: sde: handle event disable failure

If unregister event fails to succeed due to invalid params
or due to inappropriate hardware configuration, The callback function
is not deleted from the irq table. This leads to list corruption
issues in the subsequent calls to event enable and disable.

Change-Id: I549bd15b07b9a3b04c0f0a239bd85748acf7d473
Signed-off-by: Lakshmi Narayana Kalavala <lkalaval@codeaurora.org>