drm/amd/display: Remove unused mpc1_is_mpcc_idle

mpc1_is_mpcc_idle() was added in 2017 by
commit feb4a3cd ("drm/amd/display: Integrating MPC pseudocode")
but never used.

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>