drm: Added SDP and VSC structures for handling PSR for eDP

SDP header and SDP VSC header as per eDP 1.3 spec, section 3.5,
chapter "PSR Secondary Data Package Support".

v2: Modified and corrected the structures to be more in line for
kernel coding guidelines and rebased the code on Paulo's DP patchset
v3: removing unecessary identation at DP_RECEIVER_CAP_SIZE
v4: moving them to include/drm/drm_dp_helper.h and also already
    icluding EDP_PSR_RECEIVER_CAP_SIZE to add everything needed
    for PSR at once at drm_dp_helper.h
v5: Fix SDP VSC header and identation by (Paulo Zanoni) and
    remove i915 from title (Daniel Vetter)
v6: Fix spec version and move comments from code to commit message
    since numbers might change in the future (by Paulo Zanoni).

CC: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Sateesh Kavuri <sateesh.kavuri@intel.com>
Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>