drm/i915/dp: Rename eDP VESA backlight interface functions

Since we're about to add support for a second type of backlight control
interface over DP AUX (specifically, Intel's proprietary HDR backlight
controls) let's rename all of the current backlight hooks we have for
vesa to make it clear that they're specific to the VESA interface and
not Intel's.

v3:
* Rebase

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: thaytan@noraisin.net
Cc: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201204223603.249878-6-lyude@redhat.com