drm/amd/display: Initialize a default to phyd32clk

drm/amd/display: Initialize a default to phyd32clk

[WHAT]
phyd32clk may not be assigned before used. Give it a value based
on the default from get_phyd32clk_src().

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: Matthew Stewart <matthew.stewart2@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>