drm: helper functions for hdcp2 seq_num to from u32

Library functions for endianness are aligned for 16/32/64 bits.
But hdcp sequence numbers are 24bits(big endian).
So for their conversion to and from u32 helper functions are developed.

v2:
  Comment is updated. [Daniel]
  Reviewed-by Uma.

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1550219730-17734-10-git-send-email-ramalingam.c@intel.com