drm/nouveau/disp: fix cast removes address space of expression warnings

When build Linux kernel with 'make C=2', encounter the following warnings:

./drivers/gpu/drm/nouveau/dispnv50/disp.c:134:34: warning: cast removes address space '__iomem' of expression
./drivers/gpu/drm/nouveau/dispnv50/disp.c:197:34: warning: cast removes address space '__iomem' of expression

The data type of dmac->_push.mem.object.map.ptr is 'void __iomem *', but
converted to 'u32 *' directly and cause above warnings, now
recover their data types to fix these warnings.

Signed-off-by: ruanjinjie <ruanjinjie@huawei.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220924092516.10007-1-ruanjinjie@huawei.com