add i915 power estimates to `measure` subcommand

[zx2c4]

It'd be nice if intel-undervolt measure showed the results from the i915 driver, which is in the same uj output as the current rapl source. Here's some code I just wrote to do it:

#include <stdio.h>
#include <stdint.h>
#include <inttypes.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
    FILE *f;
    uint64_t this_microjoules, last_microjoules = ~UINT64_C(0);

    setbuf(stdout, NULL);
    f = fopen("/sys/kernel/debug/dri/0/i915_energy_uJ", "r");
    if (!f) {
        perror("fopen(/sys/kernel/debug/dri/0/i915_energy_uJ)");
        return 1;
    }

    printf("Measuring...");
    for (;;) {
        if (fscanf(f, "%" PRIu64, &this_microjoules) != 1) {
            perror("fscanf");
            return 1;
        }
        rewind(f);
        if (last_microjoules != ~UINT64_C(0))
            printf("\x1b[0G\x1b[0K%.2f W", (double)(this_microjoules - last_microjoules) / 1000000.0);
        last_microjoules = this_microjoules;
        sleep(1);
    }
    return 0;
}