drm/amd/display: Initialize mode_select to 0

[WHAT]
mode_select was supposed to be initialized in mpc_read_gamut_remap but
is not set in default case. This can cause indeterminate
behaviors.

This is reported as an UNINIT error by Coverity.

Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Fangzhi Zuo <jerry.zuo@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>