drm/xe: Cleanup xe_mmio.h

We don't need <linux/delay.h> include since commit 5c09bd6c
("drm/xe/mmio: Move xe_mmio_wait32() to xe_mmio.c").

We don't need <linux/io-64-nonatomic-lo-hi.h> include since commit
54c65966 ("drm/xe: Make xe_mmio_read|write() functions non-inline").

And since commit 924e6a97

 ("drm/xe/uapi: Remove MMIO ioctl")
we don't need forward declarations of drm_device and drm_file.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240520181814.2392-4-michal.wajdeczko@intel.com