i915/perf: Do not add ggtt offset to hw_tail

ggtt offset for hw_tail is not required for the calculations, so drop
it.

Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230605193923.1836048-3-umesh.nerlige.ramappa@intel.com