drm/amd/pm: add inst to smu_dpm_set_vcn_enable

First, add an instance parameter to smu_dpm_set_vcn_enable() function,
and calling dpm_set_vcn_enable() with this given instance.

Second, modify vcn_gated to be an array, to track the gating status
for each vcn instance separately.

With these 2 changes, smu_dpm_set_vcn_enable() will check and set the
gating status for the given vcn instance ONLY.

v2: remove duplicated functions.

remove for-loop in dpm_set_vcn_enable(), and temporarily move it to
to smu_dpm_set_power_gate(), in order to keep the exact same logic as
before, until further separation in next patch.

v3: add instance number in error message.

v4: declaring i at the top of the function.

Signed-off-by: Boyuan Zhang <boyuan.zhang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>