drm/amd/display: Gamma logic limitations causing unintended use of RAM over ROM.

[Why]
Our existing logic in deciding whether to use RAM or ROM
depends on whether we are dealing with an identity gamma ramp.

[How]
In addition to the is_identity flag
a new is_logical_identity flag has been
added. The is_identity flag now denotes
whether the OS gamma is an RGB256 identity
and the new logical identity will inidicate
that the given gamma ramp regardless of its
type is identity.

Signed-off-by: Harmanprit Tatla <harmanprit.tatla@amd.com>
Reviewed-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Acked-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>