drm/panthor: Simplify getting the GPU model name

This patch replaces the panthor_model structure with a simple switch
case based on the product_id which is in the format of:
        ((arch_major << 24) | product_major)

This simplifies comparison and allows extending of the function to
accommodate naming differences based on supported GPU features.

Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Karunika Choo <karunika.choo@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://lore.kernel.org/r/20250807162633.3666310-3-karunika.choo@arm.com