drm/amd/display: remove magic numbers in hdcp_ddc

drm/amd/display: remove magic numbers in hdcp_ddc

[why]
DP doesn't have message id as the first byte of an hdcp message,
current hdcp psp unifies HDMI and DP message so that it is required
when reading DP HDCP messages in hdcp_ddc, a message id needs to be
added as the first byte of the HDCP message.
The id is currently assigned as a magic number which is not a good
coding practice.

[how]
Replace magic numbers with macro defined in hdcp headers.

Signed-off-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Ashley Thomas <Ashley.Thomas2@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>