drm/amd/display: cleanup extern usage in function definition

Smatch reports this issue
hdcp1_execution.c:500:29: warning: function
  'mod_hdcp_hdcp1_dp_execution' with external linkage
  has definition

The storage-class-specifier extern is not needed in a
definition, so remove it.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>