drm/dp: add helper for reading DP sink/branch device desc from DPCD

Change-Id: Ib67fe694e8aaed0e1c18202ba9bb5e65b83b1321
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/acba54da7d80eafea9e59a893e27e3c31028c0ba.1495105635.git.jani.nikula@intel.com
Git-commit: 118b90f3f18e733c99f0e8b98ea31a815ffc4d14
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[vvalluru@codeaurora.org: resolved merge conflicts]
Signed-off-by: Venkata Prahlad Valluru <vvalluru@codeaurora.org>