ich_descriptor.c: Simplify get_density lookups by deduplicate sw

These two switch statements look up the same thing with only the
return type as different. Therefore deduce out the density encoding
lookup from the use-case logic.

TEST=`flashrom -p internal --flash-name` on Intel Kaby Lake U w/ iHDCP2.2 Prem.

Change-Id: Ieb4e9f19f4f06becd344fe08829dabe20c3c8de0
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
---
M ich_descriptors.c
1 file changed, 50 insertions(+), 80 deletions(-)