drm/omap: Hide sparse warnings

sparse reports:

drivers/gpu/drm/omapdrm/omap_dmm_tiler.c:122:16: warning: incorrect type in argument 1 (different address spaces)
drivers/gpu/drm/omapdrm/omap_dmm_tiler.c:122:16:    expected void const volatile [noderef] __iomem *addr
drivers/gpu/drm/omapdrm/omap_dmm_tiler.c:122:16:    got unsigned int [usertype] *wa_dma_data
drivers/gpu/drm/omapdrm/omap_dmm_tiler.c:130:9: warning: incorrect type in argument 2 (different address spaces)
drivers/gpu/drm/omapdrm/omap_dmm_tiler.c:130:9:    expected void volatile [noderef] __iomem *addr
drivers/gpu/drm/omapdrm/omap_dmm_tiler.c:130:9:    got unsigned int [usertype] *wa_dma_data
drivers/gpu/drm/omapdrm/omap_dmm_tiler.c:414:9: warning: incorrect type in argument 1 (different address spaces)
drivers/gpu/drm/omapdrm/omap_dmm_tiler.c:414:9:    expected void const volatile [noderef] __iomem *addr
drivers/gpu/drm/omapdrm/omap_dmm_tiler.c:414:9:    got unsigned int *

These come from pieces of code which do essentially:

p = dma_alloc_coherent()

dma_transfer_to_p()
readl(p)

writel(x, p)
dma_transfer_from_p()

I think we would do just fine without readl() and writel(), accessing
the memory without any extras, but ensuring that the necessary barriers
are in place. But this code is for a legacy platform, has been working
for ages, and it's doing work-arounds for hardware issues, and those
hardware issues are very difficult to trigger... So I would just rather
leave the code be as it is now.

However, the warnings are not nice. Hide the warnings by a (__iomem void
*) typecast.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202407311737.VsJ0Sr1w-lkp@intel.com/


Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240806-omapdrm-misc-fixes-v1-2-15d31aea0831@ideasonboard.com