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>

drivers/gpu/drm/amd/display/dc/dml/dcn21/display_mode_vba_21.c

@@ -1294,7 +1294,7 @@ static unsigned int CalculateVMAndRowBytes(
 	unsigned int MacroTileHeight;
 	unsigned int ExtraDPDEBytesFrame;
 	unsigned int PDEAndMetaPTEBytesFrame;
-	unsigned int PixelPTEReqHeightPTEs;
+	unsigned int PixelPTEReqHeightPTEs = 0;
 
 	if (DCCEnable == true) {
 		*MetaRequestHeight = 8 * BlockHeight256Bytes;