drm/i915: Make backlight setup debugs consistent

It's confusing to debug backlight issues when one can't
easily even tell what kind of backlight control was
selected. Sprinkle uniform debug messages to all the
backlight setup functions.

Also the one that was already there (ext_pwm) was
using drm_info() for some reason. I don't think that's
warranted so switch it to drm_dbg_kms() as well.

v2: Deal with AUX backlights too (Jani)
    Move the VLV/CHV initial pipe debug there too (Jani)

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215135616.30411-1-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>