msm: mdss: hdmi: add debug method to print panel event names

Add a method to convert panel event enumerations to actual event
names in order to aid in debugging.

Change-Id: Iffdf3b974ceaf0cade50d0589a2d5014136fbd69
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>