Why is the Intel HD Graphics driver called i915?
Well, that P4 chipset is the reason for the driver name. Starting with i810, Intel outsourced the driver to Tungsten Graphics, but commissioned it as an open source one for Linux.
The first 915 chipset was released in June 2004 and soon after1, a driver for this chipset was added to the linux kernel (see also 2.6.9-rc2 changelog). The driver name was, you guessed it, i915:
+#define DRIVER_AUTHOR "Tungsten Graphics, Inc."
+
+#define DRIVER_NAME "i915"
+#define DRIVER_DESC "Intel Graphics"
+#define DRIVER_DATE "20040405"
This was consistent with previous names of drivers that supported various Intel graphics chipset families (e.g. i810, i8302). Later on, support for other chipset families (including HD Graphics) was added to the same driver, which makes that nowadays i915 supports a long list3 of Intel graphics chipsets.
1: as you can see in this message from David Airlie to Linus Torvalds and Andrew Morton
2: in fact, i830 was replaced by i915 in 2.6.39, see also the initial patch linked in another message from David to Linus
3: that list from wikipedia wasn't updated to include Broadwell & Skylake chipsets
Simply put there have been no hardware changes that force forking the driver since the i915.
All i915 and later Intel GPUs use this driver because of that. That is not to say that the driver doesn't alter it's behavior to take advantage of the newer chips, just that it's using the same core algorithms, particularly for things like memory allocation.