package/mesa3d: fix imagination Vulkan driver build

package/mesa3d: fix imagination Vulkan driver build

Since upstream commit 6e189ba6c17a2ab9b60e6fd65fc6a44a17dc9e8f, merged
in mesa-25.3.0, the imagination Vulkan driver is no longer
experimental.

Therefore, since Buildroot commit
3e296a151195f9d32dbfd62d6c75d08b822be6dd, which bumped mesa3d to
version 25.3.0, the build of a configuration such as:

 BR2_aarch64=y
 BR2_TOOLCHAIN_EXTERNAL=y
 BR2_TOOLCHAIN_EXTERNAL_BOOTLIN=y
 BR2_TOOLCHAIN_EXTERNAL_BOOTLIN_AARCH64_GLIBC_STABLE=y
 BR2_PACKAGE_MESA3D=y
 BR2_PACKAGE_MESA3D_VULKAN_DRIVER_IMAGINATION=y

fails with:

build/mesa3d-25.3.1/meson.build:4:0: ERROR: Value "imagination-experimental" for option "vulkan-drivers" is not in allowed choices: "auto, amd, broadcom, freedreno, intel, intel_hasvk, panfrost, swrast, virtio, imagination, microsoft-experimental, nouveau, asahi, gfxstream, all"

Fix this by using the proper Vulkan driver name.

There are no autobuilder failures for this issue.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>