drm/i915: Fix DPCD register order in intel_dp_aux_enable_backlight()

drm/i915: Fix DPCD register order in intel_dp_aux_enable_backlight()

For eDP panels, it appears it's expected that so long as the panel is in
DPCD control mode that the brightness value is never set to 0. Instead,
if the desired effect is to set the panel's backlight to 0 we're
expected to simply turn off the backlight through the
DP_EDP_DISPLAY_CONTROL_REGISTER.

We already do the latter correctly in intel_dp_aux_disable_backlight().
But, we make the mistake of writing the DPCD registers in the wrong
order when enabling the backlight in intel_dp_aux_enable_backlight()
since we currently enable the backlight through
DP_EDP_DISPLAY_CONTROL_REGISTER before writing the brightness level. On
the X1 Extreme 2nd Generation, this appears to have the potential of
confusing the panel in such a way that further attempts to set the
brightness don't actually change the backlight as expected and leave it
off. Presumably, this happens because the incorrect register writing
order briefly leaves the panel with DPCD mode enabled and a 0 brightness
level set.

So, reverse the order we write the DPCD registers when enabling the
panel backlight so that we write the brightness value first, and enable
the backlight second. This fix appears to be the final bit needed to get
the backlight on the ThinkPad X1 Extreme 2nd Generation's AMOLED screen
working.

Tested-by: AceLan Kao <acelan.kao@canonical.com>
Tested-by: Perry Yuan <pyuan@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200116211623.53799-4-lyude@redhat.com