dri: meson: do not prefix user provided dri-drivers-path

The user can select the location where there dri drivers
are installed by the dri-drivers-path meson option.

By default path will be $prefix/$libdir/dri.

Currently we add $prefix to the user provided path.
Resulting in an incorrect or even missing path.

v2: fixed dri_search_path by default, rebased to master

v3: new commit-message (Emil Velikov), cc mesa-stable

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109698


CC: Rafael Antognolli <rafael.antognolli@intel.com>
CC: Dylan Baker <dylan@pnwbakers.com>
Cc: 18.3 19.0 <mesa-stable@lists.freedesktop.org>
Fixes: 306914db (meson: Add dridriverdir variable to dri.pc.)
Signed-off-by: Sergii Romantsov <sergii.romantsov@globallogic.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit f6556ec7)