freedreno: fence should hold a ref to pipe

freedreno: fence should hold a ref to pipe

Since the fence can outlive the context, and all it really needs to wait
on a fence is the pipe, use the new fd_pipe reference counting to hold a
ref to the pipe and drop the ctx pointer.

This fixes a crash seen with (for example) glmark2:

  #0  fd_pipe_wait_timeout (pipe=0xbf48678b3cd7b32b, timestamp=0, timeout=18446744073709551615) at freedreno_pipe.c:101
  #1  0x0000ffffbdf75914 in fd_fence_finish (pscreen=0x561110, ctx=0x0, fence=0xc55c10, timeout=18446744073709551615) at ../src/gallium/drivers/freedreno/freedreno_fence.c:96
  #2  0x0000ffffbde154e4 in dri_flush (cPriv=0xb1ff80, dPriv=0x556660, flags=3, reason=__DRI2_THROTTLE_SWAPBUFFER) at ../src/gallium/state_trackers/dri/dri_drawable.c:569
  #3  0x0000ffffbecd8b44 in loader_dri3_flush (draw=0x558a28, flags=3, throttle_reason=__DRI2_THROTTLE_SWAPBUFFER) at ../src/loader/loader_dri3_helper.c:656
  #4  0x0000ffffbecbc36c in glx_dri3_flush_drawable (draw=0x558a28, flags=3) at ../src/glx/dri3_glx.c:132
  #5  0x0000ffffbecd91e8 in loader_dri3_swap_buffers_msc (draw=0x558a28, target_msc=0, divisor=0, remainder=0, flush_flags=3, force_copy=false) at ../src/loader/loader_dri3_helper.c:827
  #6  0x0000ffffbecbcfc4 in dri3_swap_buffers (pdraw=0x5589f0, target_msc=0, divisor=0, remainder=0, flush=1) at ../src/glx/dri3_glx.c:587
  #7  0x0000ffffbec98218 in glXSwapBuffers (dpy=0x502bb0, drawable=2097154) at ../src/glx/glxcmds.c:840
  #8  0x000000000040994c in CanvasGeneric::update (this=0xfffffffff400) at ../src/canvas-generic.cpp:114
  #9  0x0000000000411594 in MainLoop::step (this=this@entry=0x5728f0) at ../src/main-loop.cpp:108
  #10 0x0000000000409498 in do_benchmark (canvas=...) at ../src/main.cpp:117
  #11 0x00000000004071b0 in main (argc=<optimized out>, argv=<optimized out>) at ../src/main.cpp:210

Signed-off-by: Rob Clark <robdclark@gmail.com>