drm/dp: DRM DP helper for reading Ignore MSA from DPCD

DP sink device sets the Ignore MSA bit in its
DP_DOWNSTREAM_PORT_COUNT register to indicate its ability to
ignore the MSA video timing parameters and its ability to support
seamless video timing change over a range of timing exposed by
DisplayID and EDID.
This is required for the sink to indicate that it is Adaptive sync
capable.

v3:
* Fi the typo in commit message (Manasi)
v2:
* Rename to describe what the function does (Jani Nikula)

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200619212356.19285-2-bhanuprakash.modem@intel.com