meson: Don't confuse the install and search paths for dri drivers

meson: Don't confuse the install and search paths for dri drivers

Currently there is not a separate option for setting the search path of
DRI drivers in meson, like there is in scons and autotools. This is an
oversight and needs to be fixed. This adds an extra option
`dri-search-path`, which will default to the value of
`dri-drivers-path`, like autotools does.

v2: - Split input list before joining.
v3: - use : instead of ; as the delimiter. The autotools help string
      incorrectly says ; but the code uses :
v4: - Take list in pre : delimited form (Ilia)
    - Ensure that the dri-search-path is absolute when using
      dri_drivers_path

Fixes: db9788420d4bc7b4 ("meson: Add support for configuring dri drivers directory.")
Reported-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net> (v2)
Reviewed-by: Eric Engestrom <eric@engestrom.ch> (v3)