mesa/st: Lookup parameters without using names

Use the new MainUniformStorageIndex field in Parameter instead.  It
was added so we could match those in the SPIR-V case, where names are
optional.

v2: Use MainUniformStorageIndex for all cases.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com> [v1]
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>