Make SGE parameter handling more VF-friendly.

Add fields to hold the SGE control register and free list buffer sizes to
the sge_params structure. Populate these new fields in
t4_init_sge_params() for PF devices and change t4_read_chip_settings() to
pull these values out of the params structure instead of reading
registers directly. This will permit t4_read_chip_settings() to be reused
for VF devices which cannot read SGE registers directly.

While here, move the call to t4_init_sge_params() to
get_params__post_init(). The VF driver will populate the SGE parameters
structure via a different method before calling t4_read_chip_settings().

Reviewed by: np
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D7476