vpx_dsp/get_prob: make clip_prob branchless
vpx_dsp/get_prob: make clip_prob branchless

+ inline the function directly as there was only one consumer
(get_prob())

this is an attempt to reduce the amount of branches to workaround an amd
bug. this change is mildly faster or neutral across x86-64, arm.

http://support.amd.com/TechDocs/44739_12h_Rev_Gd.pdf
665 Integer Divide Instruction May Cause Unpredictable Behavior

BUG=chromium:639712

Suggested-by: Pascal Massimino <3d06a24b48294f276422da74d333eca053545d99@gmail.com>
Change-Id: Ia91823aded79aab469dd68095d44300e8df04ed2

Topopiccione/libvpx,Topopiccione/libvpx,ShiftMediaProject/libvpx,Topopiccione/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/vpx,mwgoldsmith/libvpx,Topopiccione/libvpx,mwgoldsmith/vpx,webmproject/libvpx,webmproject/libvpx,mwgoldsmith/libvpx,mwgoldsmith/vpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,mwgoldsmith/vpx,mwgoldsmith/libvpx,Topopiccione/libvpx,mwgoldsmith/libvpx,webmproject/libvpx,mwgoldsmith/vpx,webmproject/libvpx,mwgoldsmith/vpx,webmproject/libvpx,Topopiccione/libvpx