[202205] Fix issue: unconfigured PGs are displayed in watermarkstat

[stephenxs]

What I did

This is to cherry-pick #2556 to 202205
Fix issue
All the PGs between minimal and maximal indexes are displayed regardless of whether they are configured.
Originally, watermark counters were enabled for all PGs, so there is no issue.
Now, watermark counters are enabled only for PGs with buffer configured, eg. if PG 0/2/3/4/6, is configured, PG 0-6 will be displayed, which is confusing by giving users a feeling that PG 7 is lost

Signed-off-by: Stephen Sun [email protected]

How I did it

Display valid PGs only

How to verify it

Manually test and unit test.

Previous command output (if the output of a command-line utility has changed)

       Port    PG0    PG1    PG2    PG3    PG4
-----------  -----  -----  -----  -----  -----
 Ethernet0      0      0      0      0      0
 Ethernet2      0      0      0      0      0
 Ethernet8      0      0      0      0      0
Ethernet10      0      0      0      0      0
Ethernet16      0      0      0      0      0
Ethernet18      0      0      0      0      0
Ethernet32      0      0      0      0      0

New command output (if the output of a command-line utility has changed)

PG1 won't be displayed if it is not configured

       Port    PG0    PG3    PG4
-----------  -----  -----  -----
 Ethernet0      0      0      0
 Ethernet2      0      0      0
 Ethernet8      0      0      0
Ethernet10      0      0      0
Ethernet16      0      0      0
Ethernet18      0      0      0
Ethernet32      0      0      0