drm/drm_fb_helper: fix fbdev with sparc64

Recent kernels have been reported to panic using the bochs_drm
framebuffer under qemu-system-sparc64 which was bisected to
commit 7a0483ac4ffc ("drm/bochs: switch to generic drm fbdev emulation").

The backtrace indicates that the shadow framebuffer copy in
drm_fb_helper_dirty_blit_real() is trying to access the real
framebuffer using a virtual address rather than use an IO access
typically implemented using a physical (ASI_PHYS) access on SPARC.

The fix is to replace the memcpy with memcpy_toio() from io.h.

memcpy_toio() uses writeb() where the original fbdev code
used sbus_memcpy_toio(). The latter uses sbus_writeb().

The difference between writeb() and sbus_memcpy_toio() is
that writeb() writes bytes in little-endian, where sbus_writeb() writes
bytes in big-endian. As endian does not matter for byte writes they are
the same. So we can safely use memcpy_toio() here.

Note that this only fixes bochs, in general fbdev helpers still have
issues with mixing up system memory and __iomem space. Fixing that will
require a lot more work.

v3:
  - Improved changelog (Daniel)
  - Added FIXME to fbdev_use_iomem (Daniel)

v2:
  - Added missing __iomem cast (kernel test robot)
  - Made changelog readable and fix typos (Mark)
  - Add flag to select iomem - and set it in the bochs driver

Signed-off-by: Sam Ravnborg <[email protected]>
Reported-by: Mark Cave-Ayland <[email protected]>
Reported-by: kernel test robot <[email protected]>
Tested-by: Mark Cave-Ayland <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
Cc: Mark Cave-Ayland <[email protected]>
Cc: Thomas Zimmermann <[email protected]>
Cc: Gerd Hoffmann <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: [email protected]
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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