drm/i915/gt: Optimize the migration and clear loop

Move the static calculations out of the loops for copy and clear.

v2:
  Fix the loss of proper error code on emit_pte

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Thomas Hellstrom <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220405150840.29351-4-ramalingam.c@intel.com