libEGL: When driver doesn't understand P3, map sRGB-encoded P3 to sRGB

Even though the driver doesn't need to do anything else differently for
P3, it must know to apply the sRGB transfer function to framebuffer
writes.

Fixes:
	dEQP-EGL.functional.wide_color.pbuffer_8888_colorspace_p3
	dEQP-EGL.functional.wide_color.window_8888_colorspace_p3

Bug: b/117104367
Change-Id: I5a0199ad353cd52d421ce86e3dc967a72b3cba71
Merged-In: I5a0199ad353cd52d421ce86e3dc967a72b3cba71