hdmitx: using BL31 cmd to access hdmi reg

hdmitx: using BL31 cmd to access hdmi reg

PD#149289: using BL31 cmd to access hdmi reg

1.hdmitx register may be accessed by kernel, bl30, bl31, etc...
To avoid concurrence issue, all should be only one port to access
hdmitx registers.Uniform kernel access to bl31.

2.optimize hdmitx driver style.

Change-Id: Ie5ae4b8bb8a641ccd39375b9addf9a8b01dcf464
Signed-off-by: Yi Zhou <yi.zhou@amlogic.com>