radv: only enable VK_AMD_gpu_shader_{half_float,int16} on GFX9+

radv: only enable VK_AMD_gpu_shader_{half_float,int16} on GFX9+

These two extensions are supported on GFX8 but the throughput
of 16-bit floats/integers is same as 32-bit. Also, shaderInt16
is only enabled on GFX9+ for the same reason, be more consistent.

This fixes a crash with Wolfenstein II because it expects
shaderInt16 to be enabled when VK_AMD_gpu_shader_half_float is
exposed. Note that AMDVLK only enables these extensions on GFX9+.

Cc: 19.1 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>