drm: msm: dsi-staging: Add support for cmd fetch from AXI bus

Certain panels have commands exceeding 64 bytes and FIFO TPG
cannot handle this requirement. This change adds support for
fetching commands over AXI bus which addresses sending
commands greater than 64 bytes

Change-Id: I125f60066871bb0a1c9ed8264ca688a6dd21b637
Signed-off-by: Shashank Babu Chinta Venkata <sbchin@codeaurora.org>