drm/amd/display: Remove unnecessary assignments

[WHAT & HOW]
TimeForFetchingMetaPTE, TimeForFetchingRowInVBlank and
LinesToRequestPrefetchPixelData are local variables. They
are freed when CalculatePrefetchSchedule() ends and need
not clearing explicitly.

This fixes 21 UNUSED_VALUE issues reported by Coverity.

Reviewed-by: Nevenko Stupar <nevenko.stupar@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>