drm/msm/dp: check dpcd version before dsc dpcd read

Read DSC DPCD registers only if dsc-feature-enable is enabled
and sink DPCD version is 1.4 or above.

CRs-Fixed: 2325207
Change-Id: Ie2ae34a1d720d7fbbf21dbc48ed3ea57c3d5de0c
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>