drm/amd/display: fix compile warning in dml

[Why]
gcc version 5.4.0 fails compilation with:
‘PixelPTEReqHeightPTEs’ may be used uninitialized in this function
[-Werror=maybe-uninitialized]

[How]
Initialized variable explicitly with 0

Signed-off-by: Roman Li <roman.li@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>