The current version checking mechanism works fine for DSI6G blocks. It
doesn't work so well for older generation DSIv2 blocks.

The initial read of REG_DSI_6G_HW_VERSION(offset 0x0) would result in a
read of REG_DSI_CTRL for DSIv2. This register won't necessarily be 0 on
DSIv2. It can be non zero if DSI was previously initialized by the
bootloader.

Instead of reading offset 0x0, we now read offset 0x1f0. For DSIv2, this
register is DSI_VERSION, and is bound to be non-zero. On DSI6G, this
register(offset 0x1f0) is SCRATCH_REGISTER_0, which no one ever seems to
touch, and from all register dumps I'vc seen, holds 0 all the time.

Modify dsi_get_version to read REG_DSI_VERSION to determine whether we
are DSI6G or DSIv2.
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>