drm/amd/display: Fix compilation error

[Why]
The C standard does not specify whether an enum is signed or unsigned.
In the function prototype, one of the argument is defined as an enum
but its declaration was unit32_t. Fix this by changing the function
argument to enum in the declaration.

Signed-off-by: Tao.Huang <Tao.Huang@amd.com>
Signed-off-by: Florin Iucha <florin.iucha@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>