ice: Add reg_idx variable in ice_q_vector structure

Every time we want to re-enable interrupts and/or write to a register
that requires an interrupt vector's hardware index we do the following:

vsi->hw_base_vector + q_vector->v_idx

This is a wasteful operation, especially in the hot path. Fix this by
adding a u16 reg_idx member to the ice_q_vector structure and make the
necessary changes to make this work.

Signed-off-by: Brett Creeley <brett.creeley@intel.com>
Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>