package/mesa3d: bump version to 20.2.1

package/mesa3d: bump version to 20.2.1

Rebased patch 0002 & 0003, added patch 0004 to fix build error.

Switched license file to license.rst due to upstream commit
https://cgit.freedesktop.org/mesa/mesa/commit/?id=4d066836e3cf307431a74eafcc8a404ce5ccea69
"No modification to the generated results have been done."

Switched true/false configure option values to enabled/disabled due to
upstream commit:
https://cgit.freedesktop.org/mesa/mesa/commit/?id=138c003d22739b0d1e6860ed398dd511a44cde04

Remove "MESA3D_PLATFORMS += drm", this platform is now automatically
selected:
https://cgit.freedesktop.org/mesa/mesa/commit/?id=e00adef34a5ce485e2c9216a268ca05e89a5fc98

Remove "MESA3D_PLATFORMS += surfaceless", this platform is now
automatically selected:
https://cgit.freedesktop.org/mesa/mesa/commit/?id=a38e21d6683aeecb51aea4f933a77c6e7f1a6179

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>