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;
}