drm/dp: Rename the edp_sdp_header as dp_sdp_header

No functional changes in this patch.

The SDP Header is a generic header for secondary data packets for
both eDP and DP so call it dp_sdp_header. This header gets used for
different SDP types already defined.
Also header bytes 2 and 3 are secondary data packet specific header bytes.
So change the comment to indicate the same.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1524770868-16869-1-git-send-email-manasi.d.navare@intel.com