ttn: Only call get_disk_shader_cache when it is set

p_screen.h states that the function may be NULL if the driver doesn't
support an on-disk shader cache.

Fixes: 4db880d8 ("ttn: Implement disk cache")

Reviewed-by: Alyssa Rosenzweig <alyssa@collabora.com>
Signed-off-by: David Heidelberg <david.heidelberg@collabora.com>
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