drm/i915: Rename pwm_* backlight callbacks to ext_pwm_*

Since we're going to need to add a set of lower-level PWM backlight
control hooks to be shared by normal backlight controls and HDR
backlight controls in SDR mode, let's add a prefix to the external PWM
backlight functions so that the difference between them and the high
level PWM-only backlight functions is a bit more obvious.

This introduces no functional changes.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@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/20201204223603.249878-3-lyude@redhat.com