drm/i915/dpcd_bl: uncheck PWM_PIN_CAP when detect eDP backlight capabilities

BOE panel with ID 2270 claims both PWM_PIN_CAP and AUX_SET_CAP backlight
control bits, but default chip backlight failed to control brightness.

Check AUX_SET_CAP and proceed to check quirks or VBT backlight type.
DPCD can control the brightness of this pannel.

Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201009085750.88490-1-aaron.ma@canonical.com