drm/i915/: Re-work clflush_write32

Use drm_clflush_virt_range instead of clflushopt and remove the memory
barrier, since drm_clflush_virt_range takes care of that.

v2(Michael Cheng): Use sizeof(*addr) instead of sizeof(addr) to get the
		   actual size of the page. Thanks to Matt Roper for
		   pointing this out.

Signed-off-by: Michael Cheng <michael.cheng@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220321223819.72833-5-michael.cheng@intel.com