etnaviv: slim down etna_shader_key

etnaviv: slim down etna_shader_key

The max number of exposed fragment samplers and views on any hardware is 16,
so there is no point in having arrays of 128 entries to track information
for the shadow sampler lowering in the shader key. Most of them will never
be used. Reduce the size of the arrays to what is actually necessary and add
a assert to make sure the sampler (view) limit isn't bumped without a
matching change in the shader key.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18968>