fbdev: vfb: simplify memory management

Substitute home-brewed memory management for framebuffer memory with what
core mm provide us: vmalloc_32_user() and remap_vmalloc_range()

The former is designed to allocate virtually contiguous area which is
32bit addressable and zeroed so it can be mapped to userspace without
leaking data. The latter does the similar job to remap_pfn_range() but
additionally validate vmalloc'ed area and it's size.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>