SF: fixing estimatedDpi checking x twice

This was caught after the change was merged. In practice it is unlikely
that y would be -1 with x not also being -1, but code shouls still be
correct.

Flag: com.android.graphics.surfaceflinger.flags.correct_dpi_with_display_size
Bug: 361413340
Test: HWComposerTest
Change-Id: I8ed47e73239c3d1cbeca8f093644d852cfb04011