i965/vec4: load dvec3/4 uniforms first in the push constant buffer

i965/vec4: load dvec3/4 uniforms first in the push constant buffer

Reorder the uniforms to load first the dvec4-aligned variables in the
push constant buffer and then push the vec4-aligned ones. It takes
into account that the relocated uniforms should be aligned to their
channel size.

This fixes a bug were the dvec3/4 might be loaded one part on a GRF and
the rest in next GRF, so the region parameters to read that could break
the HW rules.

v2:
- Fix broken logic.
- Add a comment to explain what should be needed to optimise the usage
  of the push constant buffer slots, as this patch does not pack the
  uniforms.

v3:
- Implemented the push constant buffer usage optimization.

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Cc: "17.1" <mesa-stable@lists.freedesktop.org>
Acked-by: Francisco Jerez <currojerez@riseup.net>