drm/amd/display: Make some functions static

Fix sparse warnings:

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_hdcp.c:32:6: warning: symbol 'lp_write_i2c' was not declared. Should it be static?
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_hdcp.c:42:6: warning: symbol 'lp_read_i2c' was not declared. Should it be static?
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_hdcp.c:52:6: warning: symbol 'lp_write_dpcd' was not declared. Should it be static?
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_hdcp.c:59:6: warning: symbol 'lp_read_dpcd' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>