drm/msm/dsi: Parse bus clocks from a list

DSI bus clocks seem to vary between different DSI host versions, and the
SOC to which they belong. Even the enable/disable sequence varies.

Provide a list of bus clock names in dsi_cfg. The driver will use this to
retrieve the clocks, and enable/disable them.

Add bus clock lists for DSI6G, and DSI for MSM8916(this is DSI6G too, but
there is no MMSS_CC specific clock since there is no MMSS clock controller
on 8916).

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>