drm/amd/display: Change dc_surface_update cm2_params to const

[WHY]
cm2_params is used to set update_flags.
It's value is not intended to be modified.

[WHAT]
Change the declaration of cm2_params to be a constant variable

Reviewed-by: Tao Huang <tao.huang@amd.com>
Reviewed-by: Ariel Bernstein <eric.bernstein@amd.com>
Signed-off-by: Shunlu Zhang <Shunlu.Zhang@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>