drm/i915: Set HDMI hot plug interrupt enable for only the output in question.

We detect HDMI output connection status by writing to HOT Plug Interrupt
Detect Enable bit in PORT_HOTPLUG_EN.  The behavior will generate a specified
interrupt, which is caught by audio driver, but during one detection driver
set all Detect Enable bits of HDMIB, HDMIC HDMID, and generate wrong
interrupt signals for current output, according to the signals audio driver
misunderstand device status. The patch intends to handle corresponding
output precisely.

It fixed freedesktop.org bug #21371

Signed-off-by: Ma Ling <ling.ma@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>