drm: generic fn converting be24 to cpu and vice versa

Existing functions for converting a 3bytes(be24) of big endian value
into u32 of little endian and vice versa are renamed as

s/drm_hdcp2_seq_num_to_u32/drm_hdcp_be24_to_cpu
s/drm_hdcp2_u32_to_seq_num/drm_hdcp_cpu_to_be24

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Suggested-by: Daniel Vetter <daniel@ffwll.ch>
cc: Tomas Winkler <tomas.winkler@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/20190507162745.25600-4-ramalingam.c@intel.com