left_above_mv and above_block_mv return as_int

as_int is defined as uint32_t in vp8/common/mv.h

Cleans up -Wextra warnings:
signed and unsigned type in conditional expression
this_mv->as_int = col ? d[-1].bmi.mv.as_int : left_block_mv(mic, i);
                                            ^
this_mv->as_int = row ? d[-4].bmi.mv.as_int : above_block_mv(mic, i, mis);
                                            ^
left_mv.as_int = col ? d[-1].bmi.mv.as_int :
                                           ^

Change-Id: Ia043764e4ce93d2152d2269b1c7b28b5d5f814cf