vc4: Add headers and .pc files for VC4 userspace development.

The headers were originally written in Mesa, imported to the kernel,
and improved upon in vc4-gpu-tools.  These come from the v-g-t copies
and will replace the Mesa and v-g-t copies, and hopefully be used from
new tests in igt, as well.

v2: Fix linking against libdrm_intel instead of libdrm.
v3: Drop Libs and Cflags since they'll be inherited from libdrm.
v4: Switch to Requires.private.  I was wrong about standard practice,
    apparently only Intel was doing plain Requires (sorry to all
    involved).

Signed-off-by: Eric Anholt <eric@anholt.net>