drm/amd/display: Add a conversion function for transmitter and phy_id enums

Clang warns:

../drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link.c:2520:42:
error: implicit conversion from enumeration type 'enum transmitter' to
different enumeration type 'enum physical_phy_id'
[-Werror,-Wenum-conversion]
        psr_context->smuPhyId = link->link_enc->transmitter;
                              ~ ~~~~~~~~~~~~~~~~^~~~~~~~~~~
1 error generated.

As the comment above this assignment states, this is intentional. To
match previous warnings of this nature, add a conversion function that
explicitly converts between the enums and warns when there is a
mismatch.

See commit 828cfa29 ("drm/amdgpu: Fix amdgpu ras to ta enums
conversion") and commit d9ec5cfd ("drm/amd/display: Use switch table
for dc_to_smu_clock_type") for previous examples of this.

v2: use PHYLD_UNKNOWN for the default case.

Fixes: e0d08a40

 ("drm/amd/display: Add debugfs entry for reading psr state")
Link: https://github.com/ClangBuiltLinux/linux/issues/758
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>