video: hyperv: hyperv_fb: Use physical memory for fb on HyperV Gen 1 VMs.

On Hyper-V, Generation 1 VMs can directly use VM's physical memory for
their framebuffers. This can improve the efficiency of framebuffer and
overall performence for VM. The physical memory assigned to framebuffer
must be contiguous. We use CMA allocator to get contiguouse physicial
memory when the framebuffer size is greater than 4MB. For size under
4MB, we use alloc_pages to achieve this.

To enable framebuffer memory allocation from CMA, supply a kernel
parameter to give enough space to CMA allocator at boot time. For
example:
    cma=130m
This gives 130MB memory to CAM allocator that can be allocated to
framebuffer. If this fails, we fall back to the old way of using
mmio for framebuffer.

Reported-by: kbuild test robot <[email protected]>
Signed-off-by: Wei Hu <[email protected]>
Acked-by: Bartlomiej Zolnierkiewicz <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>

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