drm/i915: drop ironlake_ prefix from edp panel/backlight functions

They now also work on vlv, which has the regs somewhere else. And
daring a glance into the looking glass it seems like this
functionality will continue to work the same for the next few hardware
platforms.

So it's better to just remove that misleading prefix and have a bit
shorter code for better readability.

The only exceptions are the panel/backlight functions shared with
intel_ddi.c, those get an intel_ prefix.

While at it make the vdd_on/off functions static.

And one straggler was missing the edp_ in the name, so make everything
neatly OCD.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>