drm/xe: Get GT clock to nanosecs

Helper to convert GT clock cycles to nanoseconds.

v2: Use DIV_ROUND_CLOSEST_ULL helper(Ashutosh)
v3: rename xe_gt_clock_interval_to_ns to xe_gt_clock_cycles_to_ns

Reviewed-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Aravind Iddamsetty <aravind.iddamsetty@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>