i965/fs: Use byte_scattered_write on 16-bit store_ssbo

i965/fs: Use byte_scattered_write on 16-bit store_ssbo

We need to rely on byte scattered writes as untyped writes are 32-bit
size. We could try to keep using 32-bit messages when we have two or
four 16-bit elements, but for simplicity sake, we use the same message
for any component number. We revisit this aproach in the follwing
patches.

v2: Removed use of stride = 2 on 16-bit sources (Jason Ekstrand)

v3: (Jason Ekstrand)
    - Include bit_size to scattered write message and remove namespace
    - specific for scattered messages.
    - Move comment to proper place.
    - Squashed with i965/fs: Adjust type_size/type_slots on store_ssbo.
    (Jose Maria Casanova)
    - Take into account that get_nir_src returns now WORD types for
      16-bit sources instead of DWORD.
v4: (Jason Ekstrand)
    - Rename lenght variable to num_components.
    - Include assertions before emit_untyped_write.
    - Remove type_slot in favor of num_slot and first_slot.

Signed-off-by: Jose Maria Casanova Crespo <jmcasanova@igalia.com>
Signed-off-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>