arm64: defconfig: enable DisplayPort altmode support

Enable the DisplayPort altmode, it is required to get DisplayPort output
to work on devices like Qualcomm Robotics RB5 platform.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewd-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Link: https://lore.kernel.org/r/20231011103245.96638-2-dmitry.baryshkov@linaro.org
Signed-off-by: Bjorn Andersson <andersson@kernel.org>