Fix reading power limit on Nvidia GPUs

[artur-ag]

For some Nvidia GPUs, nvidia-smi does not report power_limit, but rather current_power_limit. I updated Nvidia.php to read both these fields, with current_power_limit being given preference. This causes the powermax data field to always be N/A, and the power bar in the interface to never fill up. This PR fixes that.

There was also a bug in the code where it checked for power_limit but then read current_power_limit. I fixed that too.

Sample nvidia-smi of an RTX 2080 Ti (gpu_power_readings section):

<gpu_power_readings>
        <power_state>P8</power_state>
        <average_power_draw>N/A</average_power_draw>
        <instant_power_draw>2.42 W</instant_power_draw>
        <current_power_limit>250.00 W</current_power_limit>
        <requested_power_limit>250.00 W</requested_power_limit>
        <default_power_limit>250.00 W</default_power_limit>
        <min_power_limit>100.00 W</min_power_limit>
        <max_power_limit>280.00 W</max_power_limit>
</gpu_power_readings>