build: qcom_target: Define MSM_VIDC_TARGET_LIST

Currently, in the media HAL, we're replacing the usage
of this variable with a check for TARGET_BOARD_PLATFORM.

Instead of having to do that every time a new branch is made,
define the variable in qcom_target.

Change-Id: Idd85b6f0b167bbc64833eb03335abcaa7b9de4a9