drm/dsc: Add helpers for DSC picture parameter set infoframes

According to Display Stream compression spec 1.2, the picture
parameter set metadata is sent from source to sink device
using the DP Secondary data packet. An infoframe is formed
for the PPS SDP header and PPS SDP payload bytes.
This patch adds helpers to fill the PPS SDP header
and PPS SDP payload according to the DSC 1.2 specification.

v7:
* Use BUILD_BUG_ON() to protect changing struct size (Ville)
* Remove typecaseting (Ville)
* Include byteorder.h in drm_dsc.c (Ville)
* Correct kernel doc spacing (Anusha)
v6:
* Use proper sequence points for breaking down the
assignments (Chris Wilson)
* Use SPDX identifier
v5:
Do not use bitfields for DRM structs (Jani N)
v4:
* Use DSC constants for params that dont change across
configurations
v3:
* Add reference to added kernel-docs in
Documentation/gpu/drm-kms-helpers.rst (Daniel Vetter)

v2:
* Add EXPORT_SYMBOL for the drm functions (Manasi)

Cc: dri-devel@lists.freedesktop.org
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Acked-by: Sean Paul <seanpaul@chromium.org> (For merging through
drm-intel)
Link: https://patchwork.freedesktop.org/patch/msgid/20181127214125.17658-5-manasi.d.navare@intel.com