drm/i915/gvt: Make function dpy_reg_mmio_readx safe

The dpy_reg_mmio_read_x functions directly copy 4 bytes data to the
target address with considering the length. If may cause the target
memory corrupted if the requested length less than 4 bytes. Fix it
for safety even we already have some checking to avoid this happen.
And for convince, the 3 functions are merged.

Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>