agxv: Improve meson configure error message when asahi gallium driver isn't configured

Currently, if you attempt to configure agxv without the asahi gallium driver, you get the obtuse error message:

src/asahi/vulkan/meson.build:96:2: ERROR: Unknown variable "idep_agx_pack".

this is because that target is configured in the gallium driver then used in the vulkan driver. The simplest fix seems to require configuring the gallium driver when building the vulkan driver, following the example of swrast/softpipe.

The other potential fix is to move that target elsewhere, but since it seems unlikely that it would actually be desirable to build the vulkan driver without the GL driver this seemd like the more reasonable approach. With these changes, the new error message is the much clearer

meson.build:308:2: ERROR: Problem encountered: asahi vulkan requires gallium asahi

which also happens much earlier in the configure process than the previous obtuse error.