Support building amdgpu without libradeon

Building amdgpu currently requires libdrm_radeon:

meson.build

_libdrm_checks = [
  ['intel', with_gallium_i915],
  ['amdgpu', (with_amd_vk and not with_platform_windows) or with_gallium_radeonsi],
  ['radeon', (with_gallium_radeonsi or with_gallium_r300 or with_gallium_r600)],
  ['nouveau', with_gallium_nouveau],
]

Note that both amdgpu and radeon check with_gallium_radeonsi, which leads to libdrm_radeon becoming a dependency of amdgpu, which is unnecessary.

This prevents ChromeOS from building only the libraries necessary for amdgpu support, since it must also build unnecessary libraries such as libdrm_radeon:

mesa-amd-9999: Run-time dependency libdrm_amdgpu found: YES 2.4.110
mesa-amd-9999: Run-time dependency libdrm_radeon found: NO (tried pkgconfig)