vp9,encoder: fix some integer sanitizer warnings

vp9,encoder: fix some integer sanitizer warnings

the issues fixed in this change are related to implicit conversions
between int / unsigned int:
vp9/encoder/vp9_segmentation.c:42:36: runtime error: implicit conversion
  from type 'int' of value -9 (32-bit, signed) to type 'unsigned int'
  changed the value to 4294967287 (32-bit, unsigned)
vpx_dsp/x86/sum_squares_sse2.c:36:52: runtime error: implicit conversion
  from type 'unsigned int' of value 4294967295 (32-bit, unsigned) to type
  'int' changed the value to -1 (32-bit, signed)
vpx_dsp/x86/sum_squares_sse2.c:36:67: runtime error: implicit conversion
  from type 'unsigned int' of value 4294967295 (32-bit, unsigned) to type
  'int' changed the value to -1 (32-bit, signed)
vp9/encoder/x86/vp9_diamond_search_sad_avx.c:81:45: runtime error:
  implicit conversion from type 'uint32_t' (aka 'unsigned int') of value
  4290576316 (32-bit, unsigned) to type 'int' changed the value to
  -4390980 (32-bit, signed)
vp9/encoder/vp9_rdopt.c:3472:31: runtime error: implicit conversion from
  type 'int' of value -1024 (32-bit, signed) to type 'uint16_t' (aka
  'unsigned short') changed the value to 64512 (16-bit, unsigned)

unsigned is forced for masks and int is used with intel intrinsics

Bug: webm:1767
Change-Id: Icfa4179e13bc98a36ac29586b60d65819d3ce9ee
Fixed: webm:1767