ACPI: repair nvidia early quirk breakage on x86_64

x86_64 nvidia_bugs() broke when we bailed out on not finding the HPET.
However, the quirk works by checking for not finding the HPET...

Delete the nvidia_hpet_detected flag and simply test for
not finding the HPET, which is simple to do now that
acpi_table_parse returns 1 on failure.

Signed-off-by: Len Brown <len.brown@intel.com>